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:
- remove availability of labtext from most spells with only a few basic exceptions (like Aegis or healing spells) - the fact that a mage would need to spent two, three or possibly four seasons to invent a spell will significantly reduces his progress
- don't use lab customisation rules from covenant, only allow basic improvement like quality and safety - bonus stacking will again slow down powerful enchanted item and labwork in general
- limit familiar to regular animal, no magical animal and make them learn magic theory only through experience or direct teaching from the mage
- basic restriction of virtus
- dilute skill-focus by offering plethora of opportunities and choices: knowledge of many mystery cults, but each takes time to get initiated (setting lower limit in mystery cult lore before being initiated into virtues).
- availability of good tractatus and any restriction affecting training/education.
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.