Interesting. To me, the FFM aspect of Theurgy was always more of a secondary benefit. The biggest advantages of Theurgy over other methods are twofold.
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Daimons. It's the only way to summon them, short of making a trip into their realm. As they can use aspects, trying to use any other method will result in them just blowing up said aspect and probably coming to stomp on you later if you've annoyed them enough. Any daimons get access to some pretty fun and funky magics, what with being capable of replicating ritual effects. Not all are mind you, but still.
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Spell Spirits. As wonky as they are, it's a very thematic way to get mileage out of any number of summoning oriented virtues/magics. Things like a talisman, a focus, a spirit familiar, etc. I very much have this idea of an older ex-Misc tradition that is barely integrated that has to use spell spirits for all their formulaic spells as they're simply not able to cast otherwise. Again, why I'm trying to talk all this out before pitching the idea as a whole!
Honestly, Theurgy on the whole isn't any more complex than say Sihr, or God help you, the RoP:F stuff. Shoot, probably one of the most straightforward summoning set of powers is Goetia out of RoP:I. Why they never made a comparable magic realm version I don't know, but that's a story for another day.
You say the latter is a minor thing, whereas I view it as a critical one, so thanks for bringing it up! Saga tone is extremely important, and a really quick way to see folks get grumpy, so I'd rather talk about it now so as to make sure I'm not being disruptive! That said, I don't see mystery cults coming to take a dominant position in things given all the potential story hooks. Drive the occasional story, sure, but it's likely to only affect the magus in question, so it's unlikely to suck in others/change the tone of the saga. It's no different than if you had any of the house mystery cults, or even a Tremere, a Q, etc. There are always going to be outside forces, ya know?
That said, don't worry, if my summoner concept doesn't pan out, I have an Auram focused nature lore ex-Misc or a Gifted Mercere wannabe Redcap waiting in the wings, either of whom I suspect would fit in with your view of things quite nicely!