I'm going to follow this thread too, I'm a very complex mystery case and I can't really tolerate the autonomic medications and they don't help in me being upright.
So I was diagnosed with ME (myalgic encephalomyelitis) by 3 different physicians and it does usually involve post exertional malaise which is basically crashing and getting very very sick (or at least more fatigued) after any physical or mental activity. Though to receive a diagnosis there is a huge packet your physician has to go through and there is a lot of testing involved. I.E. they look at natural killer cell count and function, B cells, T cells, cytokine levels, potential chronic infections which is also hallmark of ME but some people don't respond to the treatment but over a decade ago I tested positive for Parvo and different strains of coxsackie. I am extremely fatigued too, it's just insane.
IVIG did help me in the past, saline infusions as well, and I was just diagnosed with small fiber neuropathy but no word from the physician who ordered the biopsies. There isn't really a concrete treatment for ME and it's so complicated, there's just no funding over here for this disabling disease but many patients get help from different things sounds like you've tried some with the LDN, anti-inflammatory medications, etc.
Some ME patients have found through geneticists (genetic testing) that they have Mitochondrial disease. I don't really know the treatment for this but causes bad fatigue starting at a cellular level.