Figured out the issue.
The problem was with the way I was exporting my model from Blender. In my Blender project, I have multiple copies of my avatar - one with higher resolution for my PC Avatar and another with a lower poly version for my Quest fallback avatar.
As such, Blender doesn’t allow for duplicated object names, and so my Quest version had an armature named ‘armature.001’ and a mesh body named ‘body.001’ - and importing the model into Unity reflected those names.
However, I was renaming my armature and body to just ‘armature’ and ‘body’ so I could preserve my animations without having to rebuild them all using the new names.
The default VRC colliders don’t like that.
Everything else will work except those, and they just sort of disassociate after being referenced (as can be seen when trying to run the game inside Unity for testing).
The simple fix is just to rename the armature and mesh inside Blender, export the model and just don’t save those changes in Blender if you don’t want the renaming to ‘stick’.
A dumb mistake on my part but maybe someone else out there will learn from it. 