One thing that may help is extending your lit area. In lighting tab (can't think if field names off top of my head) change the first input to like 4, and then the second to 0.125, then apply a constant color, or scalar, to direct lighting, and set it to like 2. The clouds will have a larger glow area for overcast like IRL.
PS directly lit clouds may not be calculated like directly lit surfaces, as they are volumetrics.