I always figured that to pull off an evothaum you can assume some things are the same, an attack is an attack, and shifts generated for weapon rating and control are the same as for regular evocation. A block is a block, and same with maneuvers. The big difference is that you gain narratively different ways to create those effects. Basically combat magic will look the same as with regular casters but will have a different flavor.
The real perk of evothaum is to be able to cast rituals at basically instantaneous speed that fall outside of the usual combat options of attack, maneuver, block and counterspell. For instance you could do an Athletics skill replacement ritual at the speed of evocation. Of course to properly get that speed it would have to be a ritual that can be performed at no prep meaning it will be capped by your Lore and Lore Foci, and any Debt you're willing to accrue to make your spell go through or to ramp up its power.
It doesn't actually remove complexity at all this way, which implies to me getting more bang for your buck from an evothaum shift than a regular spell powering shift, so there isn't really anything changing except the speed at which the spell is being cast. And that particular change mostly just speeds up play without getting too overpowered.
That being the way I interpreted it, then I would allow Thaumaturgy foci to function with Evothaum since the Evothaum you'll be doing is capped, unlike regular rituals and Thaumaturgy. Even if you pull off something stupidly big with your Evothaum it's only going to be by burning Fate Points or taking Debt.
