In my experience, I agree about boulder gardens. I don't do big drops. So I wonder if I ride mostly Allaire, am I hurting my back. When I go over patches of bumpy trails, I am up on my pedals with my knees flexed. I'm pretty far past 50 with no back problems.
Allaire is my “home park”.
IMHO full squish isn’t gonna help your back there - far more important is to have the right bike fit.