First try with medium volume. I used very low density (0.01) and a high medium radius (120) corresponding more or less to the terrain size (240 meters). The result is much more natural and homogeneous.
Rendering time still good, about 15 min in 2000x2000 and 2560x1440 and only 200 samples on 3 GTX 1070. Denoiser volume really helps on this.
Modeled with Blender
Rendered with Octane Standalone v4.0 RC 2-4