I actually meant unwrapping low res and then detailing it / sculpting over / keeping original UV . Floaters if they are one sided and borrow uv from surface bellow bakes as a single surface in Blender . Zbruch decimate has keep UV option BTW
Option 4 only works if your objects/UVs are topologically similar - ie. anything present in the highpoly needs to be in the lowpoly Baking floaters or layered clothing for example would require additional setup work. Most people work with cages because it works and is relatively simple