Additional experiments with volumetric clouds, solved some glitches and managed to mix the clouds with the scene, using the depth of the scene to avoid to show clouds occluded by scene objects (like the terrain). For now I just used a simple 3D perlin noise for the clouds, in the future I plan to try different type of noise which would be more suitable for clouds rendering, like Worley noise used by guerrilla games in Horizon Zero Dawn. Textures from Textures.com and FreePBR.com