Hoping the best for you RickJP, I hope it all gets sorted out.
Having gone through similar issues with my heart and heart rate, etc. I can relate.
I had a 4x bypass in March after the angiogram I had led to 100% heart block,
I had a 4x bypass (I was on an external pace maker for 2 days) then had open heart surgery.
After the surgery, I went into Atrial Fibrillation. Went on Amiodarone and the Afib passed while I was still in the hospital.
Started cycling 3 weeks after the surgery, and after I got stronger, when I would hit around 130 BPM I would go into rhythm related heart block, basically I would go from 130 to 75, or every other beat. Then back to a normal rhythm in about a minute or so.
First my Cardiologist took me off Coreg and it got better, I could hit say 140 before the rhythm related heart block kicked in.
Then he took me off Amiodarone and eventfully the rhythm related heart block went away.
That lasted for 4 months or so.
Then I started having PVC’ -s Premature Ventricular contractions, so the Cardiologist put me back on Coreg 3.125 twice a day.
That did not do much so he increased the Coreg to 6.25 twice a day, but warned that this could bring back the rhythm related heart block.
Yep, it did, on a 50 miler last Saturday, after the last climb of the day, at around 30 miles, my BPM went from 150 to 77.
Had to have the group wait until the episode passed, about 1 minute.
Finished the ride, called the doc on Tuedsday, and he backed off the Coreg to 3.125 2x a day,
I rode last night and hit 151 BPM, and so it does seem better.
To me the most important piece of equipment I own is my HRM, I monitor my BPM very closely and I know not to push it.
I believe you are doing all the right things.
Good luck!