Well then why would you go with a 90qt over a 41qt? You don't need 12" of height. These are terrestrial snakes which means they do not climb.
Ball pythons are shy snakes that rarely leave their hides. It is completely normal to never see them out and about. They are happiest when they are hiding. A happy ball python is a hiding ball python. If your snake is out roaming or looking for an escape route then it is probably stressed. If you got this snake as a display animal, then I am sorry to say this but they are simply not display animals.
Two tight fitting hides and a small water bowl should be able to fit just fine in a 20g. I personally don't think there is any reason to upgrade. If anything, you will just stress the animal out.
At 390g that snake could still fit comfortably in a 10g. I have my snakes under 500g in 15qt tubs.