I must say that looks phenomenal, and very much like the mountains of the original Doom.
I just wanted to mention that whenever I have set the steepness setting to 'flat', the outside area boundaries are usually the same height or better as the indoor areas, with even less fold variation that it normally can be. On some maps it does a relatively good job at making some good looking boundaries if the map a primarily outdoor style first, then indoors secondary, if still set to 'flat'. Have you looked into this any?
Also, have you since looked into how boundaries can also host decor like the trees, gore, or etc. like V6?