My symptoms and test results sound very similar, with extreme fatigue, post exertional malaise, and brain fog. I take a beta blocker to help even out my heart rate and blood pressure going from lying down to standing up (after a tilt table test). Low dose naltrexone helps a little with fatigue. What helped the most with fatigue: I started very slowly with building up exercise/activity, starting with only three minutes every day on a recumbent bike a year ago to now an hour three times a week. That slow build up kept me under my fatigue threshold and has built up my capacity for physical and cognitive activity.

The most that has helped me with brain fog is guanfacine and N acetyl cysteine at night (there's a published case study on this that I shared with my doctor), and the supplement lions mane. I participated in a clinical trial that helped, but the side effects were rough. I've found that all of these small things, including breathing exercises and reducing stress, have added up little by little and day by day to at least make all of these symptoms tolerable!