It may seem to you that this is some kind of effect, but in fact it is a story of the struggle against sampling and new technologies (Nanite / Virt. Shadow maps / Lumen). Next step: 5.2 project with Strata.
Final version
First iteration
A bit of Nuke grading
Unreal Scene
Gaea -> Houdini -> Static Mesh.
Gaea has the ability to save the mesh, but I like to extrude the polygons from the heightmap so that I have more space for mesh optimization