Thundercloud clouds are one of the hardest things to achieve. Did you search for 'thunderclouds' here? There might be samples, or advice in older threads. Great clouds have been a constant queeste.
I would say, you should start out with generic v3 clouds. They are the nicest and will keep their shape if you lower them (if you tick the box 'move textures with cloud'). Try a negative noise variation, like -0.3 or -0.4, no base whispyness or softness, and hit seed until you have a desired shape, then try subtle changes in buoyancy, clumping, etc. Increase edge sharpness too.