Yes, I have tested Kosvid’s approach and it indeed answers to what I was looking for: A Terrain textured (t1)+ a road textured(t2) + Dunes textured (t3) + Dunes covering parts of the road.
I have to swallow everything and look closer at Sand mask ersatz suggestion “There is currently no hard mask for sand. To make one, use “Sand” with Threshold (0.0001 works the best)” and might fill a feedback ticket in the app so that Kosvid’s (good) voice isn’t a single one anymore ! 