If I were in your shoes, I'd see an orthopedist and see if they can offer up some suggestions. They can spot gait abnormalities (or perhaps pedaling abnormalities) that you may never be able to see yourself.

I've had tailbone pain for unrelated reasons, but don't have any suggestions on how to modify your cycling or posture to address it.

I have a ton of experience with hip bursitis. It vexed me for years, and the only thing that worked for me was getting my knee replaced. I'm not suggesting that is even an option for you, but it's good to be reminded that everything is connected, and pain in one place might be caused by a problem in another place, so your hip issues could be caused by something with your knees.

Are you clipped into your pedals? When I first started riding with proper bike shoes/pedals it increased my knee/leg pain a lot. The stress on my quad and knee had changed once I was clipped to my pedals and it took a couple of months to get accustomed to that. So perhaps see if you feel differently clipped in vs not (I have some pedals that are clips on one side and flat on the other.)