In case this might resonate with someone but from the 100s of tests i've done, the most significant result for me was a 5x normal renin, which led me to eventually try fludrocortisone, the first time i finally felt 100% relief but only for a couple of days before sliding back into fatigue...
Count yourself incredibly lucky OP that you both got a diagnosis and also have such support around you. It's tough out here
I saw a close friend decline from a 90%-ile athlete to having trouble getting out of bed within a span of 2 months, and it was heartbreaking. This is someone I looked up to in the gym, went climbing with, went on hikes with. Living in Spain, he did have a hard time getting a proper diagnosis, with doctors eventually settling on ME/CFS. It's been 2 years now and he is slowly on his way back to being functional; went back to his job and he can even do light exercise once a week now.
I am going to ask him if he was prescribed corticosteroids as part of his regime. I do remember during his early tests (I know because I accompanied him) that he did test positive for Mononucleosis antibodies, but he doesn't remember showing any symptoms at that time.
My current line of research is into treatment for that, I've tried all the usual antivirals but they don't do much, but there's several new actual vaccines being developed which I'm trying to get into the trials for (in Australia).
Would be keen to know more about his story, thank you