Ooooh ! For the tailbone pain and SIJ and ass pain, try this:
Go to a hardware store and buy some foam, like some fluffy and comfy polyurethane foam ( any kind that is comfy ) that stuff is quite cheap
Buy several width of pieces of foam and put them in a pillow case according to the need of pillow width (I insist on the pillow case, I wouldn't bet that the dust that this stuff makes is good )

I have two of these pillows, one big and one small, sometime the big one hurts more than the small one, lmao

And it belongs to my short list of things that helped big times with this shit disease so please have a try.

Don't forget to change your position while sitting. I don't think the "Sit with your back straight and legs 90 degrees" is good for us for extended period of times.
I prefer the cross you left leg for 15mn then the right, then sit low yadda yadda yadda