Even if we don't have enough data for a causal link, from the information you've provided, it sounds like the elevated heart rates were more correlated with your poor sleep schedule than with your COVID infection.

Also consider that taking on strenuous exercise when you haven't done it in a while is going to cause elevated heart rates, beyond what your previous normal heart rate was during that exercise. Regular exercise will also eventually lower your resting heart rate. I think "starting a new strenuous exercise regimen" (because yes, after a year's break, that counts as "new") would also be unsurprisingly linked to elevated heart rates, even resting.

Certainly the burning sensation in your chest area is concerning, but did you rule out the more simple and common cause: acid reflux?

I agree that it's unfortunate that there hasn't been enough research on consumer health monitoring data, but I don't think your story says much about the cardiologists' diagnostic process. But I also expect that, with the general lack of understanding of the long-term effects of COVID, many doctors are not equipped to diagnose lingering health issues stemming from an infection.

But IANAD(octor), so what do I know.