My brother used to blow dry my nieces bottom with a hair dryer to make sure she was 💯 dry before putting a new nappy on. She didn’t get a rash even once
Maybe you've already tried this but i mix 1 part witch hazle 1 part pure aloe to 2 parts water into a spray bottle and shake it well. Use it to spray BEFORE wiping to be gentler on skin and AFTER and let it air dry then Aquaphor. It has been a god sent.
Let her air dry a couple times a day. Like be naked for even 5 min at a time. It works wonders. Also, water baths every day. You don’t have to use soap every time but just to clean her bottom