don't know if some of the solutions i've found would work for you or not.

most ergonomic chairs are terrible for me, i suspect its because i'm fairly small (5'1") and so they just aren't proportioned properly, but maybe there are other factors.

i wonder if the exercise ball chair might be an option? or my chair (its a foam cushioned "seat" on a spring that is rounded on the top slightly so my weight rests on my sitz bones).

i've also found that if i sit with my legs crossed (one on top of the other), it puts a lot of strain on the tendons or ligaments? that connect muscles to the trochanter part of my leg (near where the bursa are) and if i sit with my legs slightly bowed that pain is a lot less.

i'm guessing these things won't help you, but thought i'd mention them just in case.

also wonder if this cushion could be useful:

tush cush

or this thing:

back joy


sue

Spondyloarthropathy, HLAB27 negative
Humira (still methylprednisone for flares, just not as often. Aleve if needed, rarely.)
LDN/zanaflex/flector patches over SI/ice
vits C, D. probiotics. hyaluronic acid. CoQ, Mg, Ca, K.
chiro
walk, bike
no dairy (casein sensitivity), limited eggs, limited yeast (bread)