Bump normal strength, or mix a bit of diffuse to a color ramp to bump. The blend files called "scattering" shows how an artist can take these rocks from a simple mesh, to a more high res cinematic look by scattering lichen particles or algae, and filtering the diffuse with a gradient ramp to make it an ideal bump map approximation. You can copy paste the bump conversion node setups from the scattering file, it will work on all other rocks.

Use the Vcol  (vertex color) meshes. For the Vcol-only version, I removed the textures so that this will fit neatly on your Ipad/Android under 90mb.

I embedded all textures files inside, so you can use the blender scene as containers for the rock collection, and append from there. If you prefer, you may choose to store only the GLTF files.

It ranges from 2K to 8K, but most are in 4K.