I've similarly been grasping at straws to find some way that makes the fatigue go away, at least a little (although not for myself). As doctors won't prescribe any medication off-label, I've mainly looked at other methods and nutritional supplements. Here's a list (which I've been meaning to braindump for a while now anyway):

- Like you already described, monitoring heart rate variability and associated 'body battery' with a smartwatch.

- Make a log of good and bad days, note the specific symptoms. See if there are any patterns to be found that you perhaps wouldn't notice otherwise.

- Read up about POTS / orthostatic intolerance and dysautonomia in general. The book / guide "The Dysautonomia Project" is a great read. Do a simple standing test [1] a few times (when you're feeling good and bad) to see if your heart rate increases and keeps increased, even if it does not meet the criteria for POTS.

- Do breathing exercises, humming / vocalization, ear massages and meditation to activate your vagus nerve. That might help with dysautonomia. There are lots of videos / guides to be found.

- Increase salt intake if that is not a risk factor for you. That might help with orthostatic intolerance. Find a good balance of different salts in the CFS community.

- Keep light walking and (if you can) light strength training as much as you can without triggering too much PEM.

- Be outdoors in the sunlight. There are many small studies showing a correlation between getting better from CFS and sunlight. An infrared lamp might help a bit as well, but don't buy into the fancy fads. I personally prefer a simple infrared bulb because of the warmth it gives which is great in winter on its own. It's also more similar to the sun with a continuous spectrum (although relatively low intensity). Many studies emphasize illuminating your brain.

- Be aware that food supplements will probably not help too much and cost quite a bit. But you might get lucky and find something that helps. It's hard to separate correlation from causation though. It might also feel rewarding that you're busy trying something. The placebo effect might help similarly.

- Get your ferritin levels checked, and a some other basic tests around CFS as well. See if your ferritin level has been recorded in the past as a baseline, because the one-level-fits-all approach is flawed (and especially for women). 15 (women) or 30 (men) µg / L is probably too low, even if that's considered 'normal'. Lactoferrin might help your body regulate iron levels and keep pathogens from using iron, but the scientific evidence is pretty weak.

- In case of deficiencies supplement with amino acid or organically bound metals, for example iron bisglycinate. Something like Thorne Basic Prenatal at 1/3 or the recommended dosage (1 pill per day) is quite cheap with many nutrients that might help a bit in a form that absorbs well.

- Creatine is widely used in sporting performance enhancement by facilitating ATP recyling and acting as buffer. Because it's so widely used it seems very low-risk and applicable to CFS. There are some very small scale studies reporting positive effects.

- Other things that are doubtful to help, but perhaps worth trying: wide spectrum probiotics, NAD+, D-ribose, nattokinase / lumbrokinase (but be careful and use a small dose), NAC, ALC.

[1] https://www.standinguptopots.org/resources/diagnosing-pots