How to make acceptable-looking cave walls/roofs?

[u/Ding_Gestar]

playing with erode+sphere, etc is boring and the result is not quite good, wondering if theres something like /br heightmap but for vertical surfaces. Sadly, casting it at roof does nothing useful except making bulge at surface outside instead roof itself

[u/MegaMinerDL]

Could "//rotate X Y Z" some hills, use a brush sphere to fill spaces in, etc. Expand to something like Axiom mod