Hi there and welcome.

As I read your post I thought this could be a post from myself.
Most of your symptoms are so much like my own it's not funny.

I started with knee problems when I was 10 dx with osgood schlatters. My hips would pop in and out and had feet problems. My lower back started to play up in my late teens ever since things have gone down hill. Shoulders, sternum most joints affected. My spine is a mess with much degeneration curvatures, stenosis and lesthesis. CT of SI's only shows mild changes. I also have cysts in a number of places including lower back.

However we differ in that I have been dx with AS I am also HLA B27 +.

I am no expert but for me your problems do sound very much like they point towards AS.
Don't give up the fight, keep pushing your docs for an answer. It took me well over 10yrs to be diagnosed.
Luckily I have found a lot of relief in LDN (low dose naltrexone) Although I did forget to take it a few nioghts and I am realy paying for it today.
Keep researching and reading here. I may also be of benefit to try the Low starch or no starch diet. Details can be found on this site. It didn't work for me but many others have found it great.

Again welcome and all the very best. Hope you find some answers soon.

Trish