i would say as feedback the main performance issue might be with voxel gi since maybe meshes are set to dynamic gi mode and for some reasson that kills fps, if it not working try lightmap gi since it now works far better and might not crash now. Also the frenes can be fixed by used the shader baker from 4.5 , though it increases load times
The 2 issue might be with shadows which godot devs know and plan to optimize and potentially add ss shadows in 4.7(clay said so at godot tomorrow.
Else it might be the depth prepass