I was in a somewhat similar situation with my last mare - increasingly irrational behavior under saddle and abnormal repro findings that were never conclusive. Her behavior was definitively tied to her repro cycle though, which it doesn’t sound like your mare’s is. Like one of the above posters, I elected to spay, and while it didn’t give her back a riding career, she is happily retired as a tease mare.
I will also say that I am incredibly empathetic for all you have done for your mare. I completely understand that feeling of just wanting to find something wrong. There is a tendency on these types of threads to assume the worst from the poster - that their vets aren’t competent, that the owner hasn’t tried to find a cause of pain, that anything not mentioned means it wasn’t done or was ignored - and then act shocked when that person starts to get a bit defensive.
It is easy to criticize a response based on a summary posted on COTH, but often I think the responses forget to take into account that the poster has been knee-deep in dealing with the issue for long enough that not all the information is going to get captured in the immediate post. Some always ask for video, repeatedly, but seem to lose sight of the fact that they are asking a stranger on the internet to post video of their horse in its worst moments, generally after a few posts insinuating they have ignored that their horse is in pain.
Good luck with your mare!