All of this was done just with CSGs. The height range on most of these planets was actually set to 0, so without the subtractive brushes they're all just smooth spheres. The biggest tool I used was the crater CSG - you know, the little pockmarks that generate in the moon biome. If you copy one of those and scale it up massively, you can carve out massive sections of terrain in a way that is compatible with unit pathfinding. Sink them into the ground a bit and you can create gaps under the top-level terrain. It really all comes down to trying things out and seeing what happens. Don't be afraid to break everything, you might learn something in the process. (Just save frequent revisions!) I've linked an image of the metal planet in edit mode on the /r/planetaryannihilation post, you might want to check that out.