I think it's a pretty excellent image already! That foreground cracked salt flat surface shading is superb IMO. It looks rather like Death Valley or the Black Rock Desert. I agree the clouds aren't quite there yet, but not bad either. Honestly I'm not sure you need the 2 layers, and 4 definitely seems like it would just fill up the sky to not much benefit. I'm not sure if you're trying to emulate the clouds in the reference image as well as the ground, if that's the case then my thoughts may not be relevant, but personally I think the impact of this scene and overall aesthetic would be enhanced by *less* clouds, not more. I'd be tempted to see what it looks like with just the lower, bigger clouds, or if you do keep the upper ones then very thin and patchy there.
Anyway I think without much tweaking this could be a superb finished scene.
- Oshyan