My rock slab got up about 7 million polys in ZBrush, and Blender was struggling. Especially as I have a scene that will require about a dozen of them. So normal mapping seemed the only option, but trying to bake a normal map to a new cube of about the right size didn’t work too well.
Then I remembered how we solved this problem in the Photogrammetry workshop I did a few months ago. Duplicate the object and run ZRemesher on it to produce a lower poly version, which is not too high to UV unwrap pretty easily. Then bake the normals to that. So here I have a couple of slabs (in Vue), each about 24,000 polys, normal mapped. Not great, but not too bad, and quite manageable.