A couple ideas on the verge. One would be to put a color adjust on the road_and_shoulder_fade mask. That mask does a gradient from the center line out to the edge of the shoulder. With the right black and white points you should be able to create a mask a little larger than the road. Another idea is to re-run the whole thing in a different output folder with a larger road width, and use that larger mask with your original terrain. I'll add some pictures of the different masks to the docs, and we can add more masks if needed.
The latest code has options for choosing which masks to export, which should save some time. Since you have the prerequisites installed, you should just be able to re-download the new code and run it. The heightfield is still re-processed every time, maybe I should make a separate Create Masks button.
I haven't been able to figure out how to export a good .svg out of Photoshop either. It's strange, usually my problem is that Affinity is lacking a feature from Photoshop, not the other way around.