I've tried disabling them by setting their spawn to 0 but crashes still continue. This is the best monster mob out there.

crash-2020-10-28_04.34.58-server (1).txt

Did you try re-download the jar file again? It seems that you're using the old 1.2.1 version.

Also try setting Avaton's spawn rate to 0 instead of Banshee's

crash-2020-10-31_17.18.24-server (1).txt

So now its the wendigo? Which is very unfortunate to disable two mobs from this mod considering each and every mob in this mod are amazing and it sucks not to hang out with them.

Pretty sure that's nothing to do with the Wendigo.

similar issues:
AlexModGuy/Ice_and_Fire#2095
HellFirePvP/AstralSorcery#1413

You can find the same error message:
at com.performant.coremod.entity.ai.goals.CustomPriotizedSlowedGoal.func_75250_a(CustomPriotizedSlowedGoal.java:52)
at com.performant.coremod.entity.ai.CustomGoalSelector.func_75774_a(CustomGoalSelector.java:355)

Mmmm so are you suggesting I remove performant and re-enable those two mobs back? Cause I'll do anything for you mod.

Avaton's issue is already fixed in the latest 1.2.1 version, be sure to download again from the Curseforge.

As for Wendigo, I believe remove performat would do.