Spondylitis is like what you describe. It is a progressive type of autoimmune disorder. In the early stages it is hard to detect the source of the inflammation. The damage that is done takes time to reveal itself. Reactive arthritis is just one type of spondylitis. The "reactive" part of the diagnosis is a reaction to an infection but the infection can be long gone when the arthritis part of it begins.

Spondylitis is an umbella term for about 6 different types. People test negative for RA.
https://www.medicalnewstoday.com/articles/spondylitis#types
You can have characteristics of all of the types of spondylitis. A diagnosis is based on which characteristic predominates. Sometimes people start out with one type and are later diagnosed with another type.

Spondylitis is a seronegative inflammatory arthritis
https://www.arthritis.org/diseases/more-about/what-type-of-ra-do-you-have
Spondylitis and PMR have similar symptoms:

PMR - involves tendons, muscles, ligaments, and tissues around the joint, symptoms often include pain, aching, and morning stiffness in the shoulders, hips, neck, and lower back.

Spondyloarthropathies – This group of rheumatic diseases principally affects the spine. The tendons and -ligaments around the bones and joints become inflamed, resulting in pain and stiffness.

I didn't have too much stiffness when I was first diagnosed 35 years ago with reactive arthritis. I was able to carry on normally when I was in remission. I just needed to do the high dose of prednisone followed by a fast taper to get back into remission. I had very high inflammation markers but not as high as yours.

The pain caused by reactive arthritis was sudden onset and it was much worse. PMR came on gradually and slowly became more painful. I didn't have enough prednisone to take so there wasn't much I could do about it.

In my opinion, PMR doesn't respond to prednisone as well as people claim because you have to take prednisone for a long time. My diagnosis of reactive arthritis with uveitis was called "steroid responsive" and I could easily go back into remission.

There is a genetic marker called HLA-B27 which might be something to test for.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1079257/
A positive test won't confirm anything but it will suggest that you are predisposed to certain autoimmune problems. HLA-B27 isn't associated with PMR but is associated with many other things.