Sorry to hear you’re going through this. I concur with Thomas. Since there isn’t much of a downside to a trial of iron supplementation, I would recommend that for a few weeks. Dose and duration can be guided by your doctor, of course. I believe most folks without anemia, even with borderline low ferritin have less fatigue after a brief course of iron–this may be in part placebo. Dosing every other day can help prevent GI upset. Also, make sure you don’t take it chronically, iron overload is a real problem.
It may be overtraining, considering you were PR-ing right before. I read somewhere, maybe on UA, that once you start PR-ing in training then you have already entered the early phase of overtraining. Not sure if this is true, so don’t quote me on this. I personally would switch to work-out I am excited to do and don’t do the ones that feel like drudgery or work at this point. And for LSD I would consider switching up the sport for variety, like biking or hiking or something. You can consider additional calories, but I don’t think that necessarily works for RED-S or overtraining, unless you were severely limiting calories beforehand.
Was you thyroid levels definitely normal, or low normal? If low normal, it may be worth repeating. Chronic high volume exercise often causes an induced hypothyroidism that resolves with less exercise. This is why many elite athletes are on synthroid.
Finally, you didn’t mention Vit D. I believe this is an underappreciated cause of fatigue in the winter due to low light exposure. You can get this tested or just take Vit D in the winter. There is not much downside to doing this. The average dose is 1000U per day, with 5x or 10x that for a few days to start–review the dosing with your doctor first to confirm, of course.
Best of luck, let us know what happens if you don’t mind.