Quick test using my latest photogrammetry rock textures.
Rather than capturing full 3D rock scans, why not build a tileable texture and use it to displace meshes and create rock formations? That way all your rocks share the same texture, which can save an incredible amount of memory. Furthermore the results are still pretty realistic!
You could even imagine creating a modular set of planes, and then just apply a different texture/displacement and make a totally different environment in seconds.
You can get these textures here : https://gumroad.com/l/bxBTd