I thought I had 0 ass for years. Just like you, nothing but bone. Pants always sliding right off me because I had nothing to hold them in place. Turns out, I actually just developed Posterior Pelvic Tilt from all the compensating I did for my bad ankles without realizing it. Also that apparently I have a really high waist and wide hips for a AMAB person and I've basically been wearing my pants way too low all this time.