This is, indeed, a bit of a mess. However I’m exploring the workflow. Terrain created in World Machine (it’s just the default startup file actually), exported as a height map, imported into Blender as a displacement texture for a grid which I then rendered with a basic grey shader, lit by an HDRI. I also exported a mask for the terrain.
In Gimp I applied the mountain ‘texture’ (just a photo of a rock formation) to the rendered terrain via the mask. Maybe I should have UV’d the terrain using Project from View and applied the rock texture in Blender in the UV editor. I might be able to match the texture to the geo a bit better that way.
Had another go with a simpler terrain. Texture is hand painted in this one. Pretty simple but process worked well. Exported UV layout and painted in GIMP.