A couple of thoughts:
First, it seems as though if you introduced Rotes into Thaumaturgy, you'd need to play by rules similar to Evocation Rotes. That is, the Rote would be limited to the character's Lore, since anything higher than that would be dependant on assets that the character can't be assured access to. (Unless you simply asserted that the Rote takes an enormous amount of time, applying the 'free' +1 per scene bonus as many times as necessary.
Second, for fairly simple Thaumaturgy, you can probably get away with some risk, as long as you have a couple of Fate and appropriate Aspects held in reserve. Say I have the 5 discipline caster you used as an example and wanted to draw 5 power in a turn. Assuming I have only two Fate (and two aspects that are as yet unused for the casting), then there is zero chance of failing my roll. All I'm doing when I roll is finding out whether I spend zero, one, or two of my Fate points. Most of the time, I'll succeed anyway, and only rarely will a 'failure' cost me more than one Fate to avert.
If the spell is more complex, then I can draw five on the first exchange, then adjust based on remaining resources. If the first roll cost me no Fate, then I can go for 5 again the second exchange. If I had to spend a Fate on the first exchange, then the next exchange I draw only 3, again with guaranteed safety. And so on.