I only have my Thaumaturge at level... 14 was it? IT's in my signature. I do play Conjurer, however, and the last boss fight my LS actually won I was playing it
I think you also literally illustrated the reason why enfeebles why the way they do right now right there in your example about the Ixali. They're high-level mobs, that attack in a huge band, you're seriously not supposed to solo that stuff, but if Enfeebles were in fact as powerful as you want them to be they'd be easily soloable as a Thaumaturge.
Although speaking of it, have you tried the strategy of 'sleepga the lot, pick them off one by one with Thunder'?
Also- WHY, in heaven's name, needs the 'spells that carry the Ancient Magic spell names' invariably be DD? Why can't they be powerful support spells? Is that a legacy term issue? Because that that's pretty silly and, sincerely, a choice left to style alone, you can't say that any of the three short of Burst is not working their potential.