I managed to actually solve this issue (!).
I made a custom Hak containing nothing but a modified CEP 2.4 appearance.2da, in which I set the Creature Personal Space to 0, and normalized the hit distances and preferred attack distances.
Now creatures no longer have to path find around each other at all. They still pathfind with the environment normally, as I haven't changed anything to do with that (haven't had to), but now I can fight 50 dragons (unrealistic) with not a single hitch on the CPU.
Even though creature models are capable of overlapping if the player pulls too many, everything still works fine. This occurrence is rare anyways if a player is playing normally, and not intentionally trying to rally every creature on a map.