Thanks, Jon. Masking isn't hard at all actually and you'd know it, you just have to think displacement if painting in greys. I always strive for as little memory use as possible, so keep the maps small (4k), and refine in TG (warp or fractal warp, add a color adjust or bias scalar). Like the river here; basic soft white line, but main body is displaced 2m down through a gamma=0.5 adjustment and shores are warped and displaced down the remaining 2m. Terrain around city is from a 16-bit heightmask, around that, distance is procedural inversely masked by a warped soft simple shape.
The main issue is to paint the roads and heights in a way that roads only raise perpendicular on raises in terrain, or they would tilt.
Most work is making and placing the 100+ houses, market stalls, people, etc. I'm thinking of editing the tgd file so all imported textures are loaded only once, and asigned to parts. In an earlier file I did it by hand, opening every object and rewiring to one texture setup, but that's tedious. I wonder if it can be done in a text editor, with a simple swap.