OP, you have all my sympathies. I have sero-negative inflammatory arthritis, so like you I had every test under the sun and nothing (aside from vit D) came back abnormal. But I was exhausted and every part of my body hurt, I could hardly stand with the pain in my feet, my knees didn't bend, ibuprofen and paracetamol were no use, and no-one could tell me what was wrong.
I was eventually referred to a brilliant rheumatologist who actually listened to my symptoms. She said that, given my symptoms, it was almost certainly something in the auto-immune/RA family, that I shouldn't get too hung up on whether it was fibromyalgia/RA/ some other form of RA, that the treatment standards were similar for most of that family of diseases and she started me on treatment. She gave me a long acting steroid shot and started me on methotrexate that day. The relief from the steroid shot was almost instant (72 hours later I went for a walk with my husband for the first time in over a year), and by the time the steroids had worn off, the methotrexate had kicked in and started to work. That was 7 years ago. And in all that time NOTHING has ever shown up on my blood tests, and my rheumy still never labels what I have as RA (as I have none of the RA markers) or anything else - I have "some form" of inflammatory arthritis, so I'm treated for it!
I guess the point of my post is that the most important thing is that you get effective treatment, rather than have a name locked down, especially as many autoimmune conditions are so hard to diagnose specifically... So what has your rheumatologist proposed as a treatment plan? Or a plan to get you to a treatment plan? Do they feel able to start you on a treatment based on your previous blood work, MRI, ultrasound, symptoms? If not what are they proposing to do to move you along to that point? Can they give you a long acting steroid to alleviate the pain in the meantime? Most RA meds take about 6-12 weeks to work, and it can be trial and error to find the right one (even with a specific diagnosis).