This change would be a future one, because it is pretty heavily breaking.

1.21.1 should probably stay the same unless there's a major content update.

The Problem

Currently, BnC 1.21 uses a registration system that is relatively volatile when it comes to classloading.

The Solution

Implement deferred registries on Fabric... I'm half joking here, what i would instead actually look at is a Supplier implementing based system, which would have xplat classes similar to deferred registers and deferred holders that would register everything at a specific point in the loader impl.
I'd probably not have it be a literal holder, however, there'd be a way to get a holder from this class.