I would either remove TC entirely, or nerf only the Thaumometer to include steel. Remember that Thaumcraft loses its big usefulness after the bronze age, since most of the other cheaty stuff is covered by other things (Native Clusters might be cheaper then a macerator, but they will not get you the byproducts that you need to progress), or is very limited in flexibility (Golems, unless upgraded, are very limited in function). Or even configurable, as is the case with Magic Mirrors. As a matter of fact, nerfing Thaumcraft in general is something Azanor specifically warns against, but I'm sure he won't mind that much if we just nerf the thing that allows everything in the first place.
If we include it later, it really will not do much. Magic Forests can be disabled, and ores can be retrogenned. But I doubt the wait will be long, as it is clear he is actively updating to 1.7, as seen on his GitHub for his API:
EDIT: I also just discovered Thaumium is registered in Greg's Meta ore stuff. We could possibly change up the cheap thaumium tools to require assembling like the other tools.