Hi gravey. from Georgia in Australia . Yes I have at present an S 1 to 4 Sacroiliac joint degeneration. Wow pain is 10/10 at times. Had all the scans you named, also. But the "blocking injections" to the S1, 2,3,4 nerves told then where it was exactly. These are rather mild injections done in outpatient surgery in your lower back under mild anesthetic. Then you go home and they ask you to keep a diary. How did it feel after one hour, two hours etc. If they blocked the "hurting nerve" they can tell from this.
So I was offered three solutions.
So I was offered PRP, RFA or FUSION. PRP (50% chance of success, newish treatment ) is an injection of platelets to see if it will heal the damaged cartilage etc. RFA is Radio frequency Ablation where they kill the affected nerve with heat. And FUSION is where they fuse the joint so it will not move and cause pain.
I selected RFA which I will have shortly. As soon as C-virus is lifted.