Vladimir, an question, why don't you update the engine to an voxel engine? I mean, it opens millions of possibilities, and everyone's using it, so there is a heck lot of articles giving inumerable ways of doing a thing. Star Citizen procedural planets are made with voxels, No Man's Sky planets are made with voxels, and the list keeps going.
Yes, it would be time consuming and something pratically impossible in this stage of development, but did you ever considered the idea? Caves, and other geological structures would be plausible, 3D water and volumetric effects embeded on the own system, and it would be much more pratical since you could use an preexisting engine to do it, or other asset codes.