As ZacD suggested, first sculpt a few unique meshes.
After that, you should try to simplify the mesh even more.The Vertical tessellation is a bit too heavy imo. try to reduce it
I also suggest to combine the meshes properly. in the right picture you have a lot of faces that cannot be seen (because they are occluded by other rocks) combine the vertices of the rocks so it’s a single mesh and not multiple disconnected ones.
This reduces the vertices and gives a better lightmap/uv result
I also suggest to create a rather big cluster (2mx2m, 5mx5m, 10mx10m) and having them fitting like puzzle pieces. Having them in a puzzle-piece arrangement improves the modularity.