Used the aspects because of possible similarity to the weapons-grade entropy curse, and because it basically imposes a moving environmental hazard. Looking back, it seems that environmental hazards are not actually aspects and that seems odd to me, but I'll change that. I know you only get one free tag, that's why I have all those extra shifts at the bottom for maximum health boxes and consequences. I figured those were used to "repeat until dead." Did I misunderstand that part?
Sort of.
There's no real system here; you just have to eyeball things to work out how many shifts they're worth.
Charging the number of shifts required to auto-kill to have the swarm repeat until the target is dead is a reasonable way to get a cost for a swarm that automatically kills its target, but I didn't think you wanted that.
Environmental hazards can be aspects too, by the way. Anything can be an aspect.
As much as I'd like to wing it or use an alternate system, the issue here is I'm using this to get an understanding of the system as is. Those suggestions are good but would be more useful if I was more familiar with the system. I need to know how to handle it if a player decides to do something even remotely similar, though obviously less lethal.
"Wing it or use an alternate system" is the system as it is.
The Thaumaturgy rules rely on the GM working out a fair complexity for whatever effect the players want to use. Guidelines are presented, but for odd ideas like this one you're on your own.
I think my suggestion ought to cover similar ideas from players, as long as players are also attacking zones and not characters.