I can't speak for everyone, but the sub-D stage is the mid-point. I use that mesh to generate the base high poly, and LOD0 by removing unnecessary verts/edges. Unreal generates usable LODs 99% of the time.
I don't use Maya but almost sure you can. It has Attribute transfer that can transfer UV and vertex normals as far as I know. Arnold does have rounding corners shaders to mask hard edges . Everything else could be painted in SPainter directly on low poly for hard surface objects. So hi poly object for hard surface things…