Coincidentally, my lowest measured rate was 21bpm too. That's pretty impressive he could still exercise! That makes me think he could have been living with the total block for many months. I could walk normally, but the week before I got treated I would have blacked out just by jogging more than 30 seconds, and my kidneys were underperforming from the lack of normal flow. My decline was gradual over about 3 months, and I had a similar feeling of unexplained fatigue.

That's an interesting take on the leadless pacemaker. For lack of the atrial lead, it's just going to beat the ventricles unsynchronized. In terms of blood flow it seems like it should be perfectly fine since your husband is a beast. I would wonder about the long term effects on the heart muscles, though. Presumably, his atriums are beating at 2 or 3x the rate of his ventricles at the moment. With the leadless pacemaker, it seems like that wouldn't change much. With the chambers all poorly unsynchronized, the heart works less efficiently as a pump and so has to work harder than otherwise, and as a result can (only potentially) lead to enlargement over time to compensate.

Both with a leadless pacemaker, and a standard two lead one, only the right (for a typical setup) ventricle is getting "captured". The left ventricle beats maybe 100ms later once the bioelectrical signal propagates through the muscle tissue. That bias leads to your ventricles doing an unbalanced amount of work, and can lead to just one chamber enlarging over time. This is stuff that most patients don't need to even know about because someone in their 70s has no practical risk of being impacted by it. Someone that plans on living for a really long time yet, though, should make sure their cardiologist is checking for that sort of thing with an echocardiogram every once in a while. Once again, the good news is that they can always install a fancier pacemaker.

Not trying to imply your husband's cardiologist doesn't know what they're doing or the leadless pacemaker isn't the best solution, I'm just sharing what I learned along the way that I thought was interesting or relevant.