I haven't tried it without the Fabric mods (although I'm making this pack with Forge, so I'm not sure how that maths out due to my lack of knowledge), so that's a good call. I'll get to that now, but I'm not sure if the mod(s) causing the issue necessarily needs to be removed as once again it was running fine prior to them being updated. So, while I will definitely disable the fabric mods and test that, afterwards, I might try to narrow them down to the root cause and downgrade it to see what effect that has. I honestly would hate to lose any content in the pack for the sake of the progression, but if I have to remove something, it's not the end the of the world.
I do kind of suspect though that one of several performance mods I've shoed in is doing some damage maybe considering it's crashing the game not on launch, but rather on loading or creating worlds.