I don’t think you’ll get many Theurges joining you, actually. The Thaumatarchy is a pretty good setup for Theurges; all the other social power centers (generals, spymasters, merchant princes, high priests) are subordinate to the Theurgic caste and at least nominally under the authority of an Ennearch. It’s lent credibility by the fact that, as you say, most aspects of power in this world are dependent on Theurgy. Most Theurges can’t imagine a better system for their interests, even if they’re unhappy with one or another aspect of it. They’ll want to destroy you, not join you and take over your revolt.
But an enlightened few (starting with Sarcifer, and including his pupil the Diadoch) believe that the hegemony of Theurges has led to an unsustainable spiral in which Theurgy is seen as the answer to every problem, and the needs of Theurges overwhelm all other considerations. The logic of blood extraction takes precedence over the rival logics of trade, military strategy, religion, etc. And the system’s coming collapse will be blamed entirely on Theurges. A stable new version of/successor state to the Thaumatarchy – if it can be formed at all – will need to have more of a balance, in which Theurges are not the sole dominant force and their interests are challenged by others.
Having come across Sarcifer’s notes on the topic, Cerlota recognizes this, and since she’s the one who’ll be training your Theurges (if you have any) you can count on her to promote this perspective among them. If your vision and hers clash, she won’t try to take you over, she’ll just go look for other more favorable allies to build her new world order.
For rather different reasons, Sarcifer himself will prefer using you to usurping you. Unless and until you manage to build a revolt on a scale that actually threatens to overtake his own. 