I have found that painting fluffy white clouds or shadows or gradients is highly dependent on brush type, type of paint, how dry the paint on the brush is, how dry the paint on the canvas is, etc.
I am trying to figure out how to do this consistently. Is there a type of paint best for this? Maybe one that is chalkier somehow compared to most paints that are heavy bodied?
Yes they do, and its almost impossible to say which is “best”. There’s one that’s best for your paper/canvas, your brushes and your technique. The only way to find the best for you, is to buy a few tubes of different brands and see which ones you prefer/find easiest. And don’t worry about wasting paints if you don’t like them; stick with white and primary colours and you can use the others for different techniques. You can even mix paint of different brands, to get a balance between them.