Here's an update guys. I went to see the endocrinologist. He's a professor of endocrinology and internal medicine.
Since I started this thread I've started suspecting more and more that my POIS symptoms are down to mast cell activation, and that orgasm is just another mast cell trigger. This is based on my symptoms matching extremely well with mast cell disorders, and my triggers mostly being common mast cell activation triggers, except orgasm.
The professor had heard of POIS, and after hearing the full list of symptoms and triggers, he agreed that it's likely to be mast cell related. I've taken a huge number of blood tests, including tryptase and lots of markers of potential secondary effects of mast cell disorders. I'll hear back on those in a week or two.
I really hope it's mast cell related as then at least I know what I'm dealing with. Honestly the orgasm thing is secondary right now, my main objective is to reduce my daily symptoms.
Huuuuuge thanks to Muon for raising the possibility of MCAD. I haven't confirmed it yet, but hey, it's the most promising lead I've had for years. And I've learned a lot about mast cells and inflammation while looking into it.
If anyone on the forum have confirmed MCAD or Mastocytosis or similar, I'd love to hear about what treatment you've gotten and how/whether it's worked.
EDIT: The plan at the moment is to test for Mastocytosis first. He said if the blood tests are all normal, it can still be a mast cell activation disorder though. In the latter case, he said one option could be to take Prednisolone, a corticosteroid, for a short period of time and see if symptoms improve. Would love to hear about any experiences with MCAD and Prednisolone.