While I agree that Thaumaturgy above your Lore needs preperation and thus isn't really possible in combat, Focus Items and Refinement bonuses mean that this can effectively be quite high.
Regardless, we've decided to go with using the Time Chart to increase the complexity of Thaumaturgy when rushing things (or lowering the complexity if taking additional time). It seems to work really well by limiting the Thaumaturgist to minor rituals where it would be appropriate to do so.
