Well first of all I am terribly sorry to hear about your dad, my condolences to you and your family.
You sound like a mirror copy of what happened to me, months of problems, zero energy, feeling like I had been hit by a truck and yes, couldn't walk 50 yards to my mail box without having to sit down and catch my breath, it got so bad I actually went to bed and hoped I wouldn't wake up in the morning. I too have been through all the heart tests, x-rays, breathing tests, blood pressure and heart rate through the roof..... I finally started feeling almost normal about the end of September, I am still not 100% but then I am almost a year older too. (I'll be 75 in January) Today my breathing is pretty good, 95% O2 after walking about 100yards at a good clip wearing a mask (which I do find makes it harder to breath), my blood pressure is back down to an average of 130/70 which for me is pretty good (when I was really feeling bad it was up around 195/115 with heart rates around 100), I am back to sleeping about 7 hours a night, for a while it was 16 or more hours a day. I think I am one of the lucky ones, I seem to be getting better and stronger every day now. I hope your recovery is as good if not better than mine seems to be. Stay safe and god bless. 😊