I’m either using this addon to transfer vert positions via UVs for the given subdiv level, which let’s me import it back in zbrush as a layer, or if it’s a single shell object I sometimes just transfer vert IDs.
If you don’t care about vert IDs you can just unsubdivide the highest subdiv, that goes either way. I don’t know an easy way of preserving layers in zbrush though, I usually do that by importing them one by one the hard way.
Also, just found out that the sculpt layers addon for Blender apparently supports layer import from ZB to Blender.