One of the first things my doctor had checked was my heart. Had a complete echo cardiogram done, plus x-rays and MRI. Nothing. Blood work, nothing. Yet I still felt like crud. It’s not as bad now, but of course I sit around the house all day, rest, and generally pace myself so I don’t over exert myself.
Oddly, I had Covid twice and never experienced fatigue or a need for rest. I did experience Myocarditis but it went away rather quickly. Your symptoms sound like classic CFS or Chronic Lyme or Fibromyalgia, not Covid. The medical world is content to let you live like that especially if you are like me and on government health care. I’d be marching myself in and asking for a stress test.