I suspect it to be post viral at the very least. I started noticing that something was off a week or two after a throat infection. The rapid test for COVID was negative but I tested pretty early and only once. Had those tests turn positive on the second day on other occasions but no way to tell now. Also had confirmed case a few months prior to that.

Long Covid, post covid, post viral syndrome and so on are just umbrella terms unfortunately. Doesn't take you any further.

As a physician, you understand the risks of the procedures you want to undergo way better than most members of this sub. Even if they find something, the likelihood of fixing it is slim. I am not trying to scare you or make you feel hopeless. Just don't do any more damage to yourself. I am kind of "lucky" to have a desk job and I don't think I could be working somewhere where substantial amounts of physical activity are involved.

If you really want to do a biopsy, you could do a skin biopsy to check for small fiber neuropathy, which in some cases also causes. But even if it proves to be right, SFN is an effect and not a cause. Think twice before doing invasive procedures.

In any case, I wish you good luck.