Bake lighting to objects that have already been "Combined"

Working with some old assets imported from 2018 Unity, trouble is all my meshes are “Combined Meshs” to my scene and I don’t have the original project files. I’ve loaded the old scene and it exported/loads in the editor fine but trying to bake lights gives me this error: I am using URP.

The combined meshs is attached to 100’s of objects in the scene, is there anyway I can either simply bake lighting to my scene or split the combined mesh into sub-meshes while keeping them on referenced everywhere in the scene.

If you’ve already combined meshes using a script or a third-party plugin, then without references to the original files, I don’t think that splitting them is possible.
In your case, it would probably make the most sense to stick with the 2018 LTS for this particular project.

Sadly not an option, can you bring in the cache of baked lights from a previous project to a new one? (Baked in 2018)

That might be tricky. Last year, we integrated our unwrapping algorithm natively so that it would not rely on a third-party library. This meant that upon project upgrade you’d need to rebake your lights. This obviously does not help you in your case…