No house rules needed. If you and your players decide so, there is an easy and adventuresome way to it:
Start all your magi in the Rhine Tribunal as lowest rung Journeymen and Peregrinatores. Thus they are forced into the world and kept from founding their own covenant as long as you like.
If they leave their Tribunal, friends and mentors, this doesn't help very much for many years after.
Should you get tired of it, they may find some patrons and finally found their own covenant.
Interesting. I really know very little about the canon working of the different Tribunals in 5th edition.
You asked for house rules, so here's a few suggestions. Remove Summae with level over 10. Do not allow any starting characters to have bonus XP abilities, such as skilled parens and Magister in Artibus.
That would slow things down. Another answer is put obstacles in the magi's way, such that each season over 10 days is used up, so they get adventure XP instead of study XP.
One suggestion that was made a long time ago was to simply advance Arts on the same scale as Abilities (see the table on page 31).
That creates a drastic reduction of power, since few magi will reach a score of 15 in an Art.
Libraries can be adjusted by simply using the rules for Abilities when creating summae for Arts.
Or just just do that for Techniques. Some edge magic use that system.
Somewhat extreme, formulaic magic requires ceremonial casting. Rituals are fine, or you need a level of Ceremonial Casting mastery (HoH:TL p 99) and maybe Mystical Choreography (HoH:S p 56).
You should first look at your previous saga and see what did make the PCs powerful. Is it access to high-level spells ? is it ability to invent perfect spell for a given situation ? is it powerful enchanted items ? Stacking of boni in skills ? Was it a player or a type of player doing too much optimisation? Is it the syndrome: one spell can solve the whole story ?
What are you trying to achieve more specifically ?
Each of these results can be tackled differently:
Keep in mind that becoming powerful can also come from low-level spells with mastery.
So you might want to also address some virtues: if you slow down art progression or spell learning, if one PC has the combo of life-link spont magic and life boost, he will be a powerhouse from the get-go (this can be tackled by throwing several challenges without letting him rest, but it means he can still solve one big problem with one flashy spell). Maybe consider banning Flawless magic.
This is why identifying what made mages powerful in previous saga is so important, because you might implement changes that won't affect the outcome if you don't identify the issue you are trying to fix.
To arch on what Lee said:
I played several solo-game (you can find on this forum some examples), and what prevent my mage (who had no competition with any resources his covenant had access to) to progress quickly was frequent adventures he had to tackle himself.
Thus the conclusion is: reduce the number of qualified grogs or companions, so mages have to get their hand dirty. Maybe grogs are either just basic fighters and servants are just good at maintaining the covenant: no thief, no literate grogs, no trader, no hunter, etc. And no teacher is available to train them and acquires other skills but the mages themselves.
It will also lead to more rounded mages instead of one-trick wonder (if it was an issue in past games).
What makes mages powerful is the ability to be single-minded and not have to divert their attention on any thing but their research, ignoring social skills, negociation, survival, etc... because a servant is there to do it.
It is an indirect way to slow progress down: either a mage tackle the issue right now and waste a season or delegate to servants and it become a bigger, more costly problem later, requiring more time and resources to fix.
You might consider going as far as banning companion. It will change the dynamic of the game.
By the way, be transparent with your players about your intention and what you will implement as changes. There is nothing more disappointing to come with some expectations for a game - which is a hobby - and get frustrated. It happens enough in real life... Discuss with them, think also if these limitations affects all the mages (NPCs) and how would look like a powerful mage in this new setting ? Is it the ability to invent and cast spell of 7th magnitude in a reasonable time frame ? 9th magnitude ? Is it achieving 15, 20 or 25 in a Art ? over how many seasons/years ? Then, based on these constraints, wave into the background why is it so ? Maybe magic is so powerful that writings get eroded/corrupted over time thus mages do not have access to high level summae or tractati slowing their progress - one to one teaching is the only method to achieve the highest level. They constantly have to reinvent the wheel.
Why is there no "good" companion ? Is magical aura hostile to mundane characters ? Or does Twilight episodes are more frequent for mundane, twisting them quickly ? Maybe they can be hired for a specific mission but won't commit for any longer because they will loose their mind or become abomination ?
You can also balance the slow magical progress by being more generous with adventures and non-magic related skills experience so it is achievable to become a "mundane" expert.
If you know the type of adventure you will be running, you might even ban some spells that would spoil the fun. In my first big saga, I banned mind-reading and truth spells because I wanted to do a lot of investigation and murder-mystery type stories. I openly said to my players that because of what I had in store, such spells would be impossible to invent/access - even for NPCs. Nobody was disappointed.
And once I got more confident in my ability to handle such spells, they were allowed for the following saga.
I appreciate all the thoughts.
My first quick thought had been simply to reduce the number and quality of available books and teachers. In various places now I see people casually throwing around QL 14 tractatus (plus book learner in many cases) which feels crazy good compared to any other method of advancement especially since the feeling seems to be that a mage should expect to have access to whatever book are desired.
It does feel that lab customization, familiars, and talisman push up the feeling of power but that is much harder for me to remember compared to previous play (see below).
I haven't played ArM since the 80s (I believe I did play in a short lived email troupe in the early 90s but we never have got far with that one). Using 1st edition we ended up naturally gravitating to more or less what you describe. It's not that mages weren't incredibly powerful it's that the focus was on starting powerful but having resource restrictions that forced the mages to become generalists including things outside of Arts.
That does not seem to be the current zeitgeist at all! It might well be the result of a few people in multiple threads repeating themselves but the impression I get is that players are encouraged to specialize fairly heavily, take most the efficient virtues, and expect to be able to get any resource (with the possible exception of vis) from their covenant without putting too much thought or role play into it.
BTW, I had that feeling from well before my Short-Ranged Magic thread! 
We belongs to the same generation it looks like. It means that I learned to play RpG with one character. Full stop. When my group started Ars, we made companion, but we all wanted to play our mage, so companions were more glorified grogs than full character.
The 5th ed really provide a new experience: with the spell guideline, it is a lot easier to design spell, powerfull spell which enticed specialisation, thus longer time in lab. In the 2nd ed, Penetration = casting total, thus the more powerful your spell, the more Penetration you had, so once you reached a certain Art level, the incentive to go beyond was lesser. The cost vs opportunity was rarely worth the effort. I had the only mage to have an Art at 31 and it is just because I wanted to invent a spell to summon a dragon. Once.
Now, you can plan on designing wonderful magical items and powerful spell and the constraint is more how efficiently you will get the knowledge/resources to achieve your goal. Time is the criticial factor in most of the case, so Companions have a bigger role to play, so the mages can spend more time in their lab.
Anyway, I enjoyed every version of the game (except possibly the Realm of Reason...)
I think it depends a lot on what you want. I assume that you are happy with the starting power level, and that we discuss advancement only. You want the magi out in the world, which may suggest that you do not want fast pace. OTOH, if the pace is slow, it takes for ever to advance, whatever the house rules, so is it fair to assume medium pace?
In my experience, power driver #1 is books, #2 is lab texts, #3 is personnell (scribes) but mainly because they proliferate the first two, and #4 is vis. I'll try to curb them without actually changing rules or limiting player options.
#1. The obvious step to take is simply to rule that great communication or good teacher is rare, maybe one in a generation has one of the two. This means that most books would be in the Q6-Q9 range. Additionally, the starting covenant should have a small library.
#2. Again, the library should be limited, but PCs may write down their own spells, but if we do something about #3, this may only mean that all the magi know all the spells, without actually increasing the repertoire.
#3. Many sagas assume that staff is recruited and trained trivially, which means that all books and lab texts are reproduced, and if we assume that there are other covenants with similar resources and inclinations, it would be illogical not to trade. If, however, we let staff be hard to recruit keep, the copying capacity is severely limited. This can be done in many ways, for instance, the land is underpopulated and the noble lords claim the population as serfs, or because too few people can cope with Gifted magi, or large crews generate their own dynamics with difficult stories to untangle.
#4. Vis is obviosuly trivial to curb, and explicitly mandated by RAW. If you make permanent sources scarce and adventure sources relatively more plentiful, you get magi out and about with little effort.
Now, all of these points have story potential. The magi may have to travel and bargain to trade books and texts and recruit staff.
You may have to make good teacher a major virtue if the players insist to exploit it to the limit, but I think that is the only actual rule change which may be required. Depending of course on just how much you want to curb power.
Oddly we liked playing companions also although we would often allow people to run both a companion and a mage at the same time.
In this we differ! Efficiency bores me. I want limitations to work within. I want to play diverse and interesting people not always ambitiously striving to do bigger things. For some characters there might be a great goal that prompts a need to get more knowledge/resources but not for all or even most. I love the mystery cults that generally provide little real power or efficiency while taking time and resources. I like your other comment about getting the mages to need to do more themselves thus effectively inefficiently using XP and time. I love the idea of a mage who goes off wandering for a year or more to look for something new even if it means he can't effectively study or use his lab for that time and in the end the desired goal isn't mechanically powerful.
It's magical academia but I want to see the people puttering about in the lab or fieldwork not really doing a lot because they have magical tenure not simply the kick ass driven researchers and the people who get off on office politics.
Ultimately this is probably the answer. Making texts lower quality might also help to bring reading XP closer to other XP but making assistants of all sorts much harder to keep around (especially scribes and teachers) goes a long way to push a saga in the direction I would like to see.
BTW, I understand that many people would not want a saga that is OSR in a way with resource limits and restrictions. But now I'm off to see if I can even roughly figure out how many educated people the Order has brought into existence (because I assume that the Church and mundane areas have roughly the same number of educated people as they always would have hand in mundane history).
A big point is that if you want a low power saga, you may also want to have a lower stakes, semi soap opera style play more like Downton Abby. The magi are the equivalent of the nobles, with companions and grogs filling in roles around the Manor. The stories involved revolve around personal quirks and motivations. The key point being that the PCs do not feel they are lacking and cannot handle the adventures.
One of the best things you can do is enforce the rules as written. Go on an adventure? Then no more xp or activities for you for the rest of the season if you take adventure XP.
And yes, I agree with all of the above house rules and my personal favorites would be:
Contrast this with my 1st Crusade campaign. This is an epic save the world blow up lots of Bad and good guys along the way campaign. Adventure xp every session beyond 10xp, piles of virtues, huge rewards of vis, so much money we dont even bother to count it, and powerful books... Countered by powerful enemies. It's alot and not everyone's style.
I enjoy reading it and would likely enjoy playing in it but would never want to be a troupe styel rotating SG in that saga!
Thankyou, glad to hear you enjoyed reading about my campaign. In my case this worked so well because it is just One Player. While it is in some senses a High Fantasy campaign, it's equally high research because I'm trying to make it historically accurate. At least as much as reasonably possible.
Multiple players or rotating SGs would just not work well at all. Especially now at about 80 to 90 sessions and 2.5 years onward.
If I tried to make this a rules as written, low xp campaign, even with one player I think he would have had a much harder time enjoying the game over time. It certainly would have had a much closer to OTL historical results.
Together these two have quotes resonate with my play back in the 80s. We had a couple of real history nerds in the group and the rest of us were sort of history nerds. But we never wanted to change history. I'm not entirely comfortable saying we wanted a semi-soap opera but we wanted to play as mages (and the people around them) in Mythic Europe instead of mages determining the fate of Mythic Europe.
I think the main thing I'd change from the rules to get a low-powered saga would be to House Rule summae out of existence.
The only kind of book that could be written would be tractati. If you then increase the vis cost of tractati so it's more in line with what summae cost in Covenants, you will have a much low-powered saga, as summae are the main source of huge XP per season.
It will also make them write tractati to barter with other covenants, if you also limit a bit the amount of vis available, since they will have to continually be purchasing books instead of just buying a handful of high-level summae and be done with.
And since they can only write so many tractati, you have a wonderful set of adventure hooks, as other covenants ask them for favors as payment for their tractati.
... and suddenly Good Teacher is even more of a killer virtue than it already is ...
Lots of low QL tractai because mages are going to have to write them earlier and without having been able to read summae until they were already fairly high level. Nice. I'd probably allow summae for at least some of the non-Arts topics but keep their QL low.
As @loke says it would make Good Teacher and Book Learner even more desirable which is a bad side effect!
Considering that Summae are being house ruled away, other XP gain increasing virtues could also be removed...
Run a slow paced saga, where each session is a few days in the life of a magi, and each season changes only when ours do, four times per real time year. Your magi will remain perfectly low power. I know people who have played for over a decade this way happliy.