do you intend to fly through the clouds or have them animate?
if not then i dont see a reason why you dont just paint them onto the backplane and use some plates for distant clouds. it will give you the best control. then overlay a bit of your volumetrics in front of those. it would be the fastest and most controllable way imo.
now, this gives me an idea, it would be cool to paint a layer of clouds and then have messiah or a shader populate these strips with volumetrics based on transparency and colors of the layer… hmm.




