As soon as you take up sponsored magic, you can evothaum, and the whole thing is moot. I am also pretty open to "self sponsoring" as a way to describe someone who has become extremely specialized in something. I think Sanctaphrax has something like that in the custom power list.
For thaum in conflict, I would always give it at least some disadvantage, mostly regarding time. So if you have a thaumaturgy ritual, that you can control in one exchange, I would have the effect go off just before the casters action in the next exchange. The whole rest of the time, he has to be focused on the ritual, and his defense drops to mediocre automatically (you can't really dodge when you are standing in a circle drawing in magical energy). If he is hit for at least one shift of stress (or maybe with a maneuver), the ritual will instantly go towards fallout. Fine with small rituals, but if you try to do something big, and you are in the third exchange and have already gathered a good amount of power, it is going to be bad.
Also, I would probably require you to put up a circle aspect first. If you don't, you could offset it by increasing the complexity of the spell by 2.