What you are experiencing is called post-exertional malaise (PEM). In all likelihood, your blood tests won’t reveal much if anything, but it doesn’t hurt to have them done just to make sure there isn’t something else going on. As you are beginning to discover, many in the medical community are ignorant (some seem willingly so) about this sort of chronic illness and its causes, triggers and management.
Long covid has many ugly faces to it. Some aspects can be from damage to the body during acute infection. Others are re-activation of other, dormant viruses, such as EBV and herpes. Blood microclotting might be causing some problems. From what I can glean, the likely culprit is a persistent viral infection in parts of the body (gut, brain, elsewhere) likely triggering an autoimmune response. But so far, the root cause of the illness has not been uncovered, and perhaps our medical technology and understanding isn’t quite there yet for that to happen (much research is underway). Some terms/conditions you might want to get familiar with are ME/CFS, POTS, and MCAS. If you are showing some susceptibility to these conditions in your post-acute viral state, I’d caution that while you can’t exercise your way out of this sort of chronic illness, you can most definitely exercise your way into a more severe and long-lasting illness.
I’ve suggested some of these things previously in other posts and probably somewhere in this thread, but given that you are several months out from your infection and are experiencing malaise after exertion, you especially need to take them to heart. Rest. Convalesce for a period of time. As you eventually re-start some activity after weeks or perhaps months, revamp your activity set to avoid heavy exertion and oxidative stress. This excludes activities like running and the type of cycling to which you are accustomed. Consider things like yoga. Casual paced walking in limited amounts. Light-to-moderate effort swimming is another good one. All in limited doses, only when you are not feeling ill and/or fatigued, with ample recovery as needed. Research something called the “spoon theory” if you continue to relapse or experience PEM. Take a hard look at your diet. Avoid or minimize foods that are inflammatory. Look into pre/probiotics for your gut. If you begin experiencing MCAS symptoms, you might benefit from reducing or avoiding foods that are high in histamines.
I’ve read accounts of people improving partially or even fully from autophagy which can apparently be induced by supplements and more typically periods of limited fasting. One thing for sure, what works for one person may not for another, or even be counterproductive for someone else. Everyone is unique and has their own set of issues, so you need to research these things and more, don’t be afraid to experiment some, confer with doctors when appropriate, and find what works for you. Many long haulers have their own mix of supplements they take. Many have prescriptions for symptoms. Some go the functional medicine route. Others swear by acupuncture. Some give up on the medical community and just focus on finding their own solutions, or giving it time and rest. I could probably list dozens of different approaches that I’ve seen discussed, and for each you’ll likely find some people who will attest to the benefit it brought them.
Don’t feel discouraged if you try but don’t immediately succeed at uncovering the magic formula to get yourself back to 100% pre-covid health, because you’ll have plenty of company – including me. I’m approaching 2 years post-acute infection, and while I am definitely doing better than when I was feeling at my worst, I am not back to being able to treat my body like a machine and put it through whatever exercise and training I want. I still have some lingering symptoms and am especially cautious about overdoing things and triggering another six-months-long fatigue relapse, which I did at one point. Good luck.