@Johan G Thanks, those are great sources indeed! I knew the Cloud Appreciation Society, but that pdf is pure gold
So what seems to have confused me is that our cloud is called simple Nimbus, but actually that's just a suffix or prefix for other precipitating clouds such as nimbostratus or cumulunimbus. So which type of cloud exactly is the texture nimbus_sheet1.rgb trying to depict?
Cumulunimbus (which is what I had in mind) have quite salient structure, e.g.:
On the other hand, nimbostratus has much less structure and fluffiness, but don't look like the default clouds to me either: