I’m a fellow A Fib sufferer, I was on the last Zwiftcast talking about it.
I was first diagnosed 3 years ago when I saw similiar things to you, ie HR spiking over 200 on hard workouts, hill repeats etc.
I saw a cardiologist and did some tests, and we concluded as I had no other symptoms to carry on. However end of 2018 I started to get more breathless, even walking the dog up a small incline, and struggled to keep HR low during steady running. He had me wear an ECG holster (I think it’s called) for 3 days, that showed I was more in A Fib than out of it.
So I had a catheter ablation done on the 7th Feb, the first month after I was still getting frequent cases of A Fib ( I bought a Kardia https://amzn.to/2W4Fcwr ) which I use to check myself 3 times a day at the moment. Since then it’s really improved, I now get a real short episode once a week, and have come off the Flecainide this week. I am still on anti coagulants.
I just started a TR plan to ease myself back into training, I’ve slowly been increasing my HR and now allow it to just tip over 160bpm, but have done no max HR work at all. I hope to by about July/August.
I’m also keeping a sharp eye on my HR during exercise, to watch for the sudden blips.