+1 for the Polar H10.
I’m 57 and suffer from Tachycardia which is similar but not the same as A-fib and have found that reducing the volume of intensive training made a difference in that I haven’t triggered my racing heart beat for a number of months and my fitness is as good, if not better, than what it was before. I’m typically doing one sweet spot, one threshold and one VO2max workout per week, with the rest of the week being Endurance/Tempo
I’m not on beta blockers, so can’t add anything with regard to their effect.
I got the Polar H10 so i could measure HRV with the HRV4 Training & Logger Apps, and have found it very reliable with no periodic odd readings like i got with my TICKR