Has your doctor listened to your concerns about what is going on for both on and off the bike? Not all doctors fully understand what or why we want to work that hard to ride a bike. My G.P. would be happy if I never got on a bike again. He thinks they are too dangerous. Funny he's never said anything about driving, and I have a history of syncope. But he listens to me and we can have a two way conversation, so for the other things he does, I'm still seeing him, despite his bias against bicycling
My cardiologist seems to be more in tune to at least the idea behind aerobic and cardiovascular fitness and how it relates to cycling and other intense exercise. I'd think your cardiologist would also be so inclined too as such cardio vascular exercise is pretty much related to their field of expertise. So maybe there is a communication breakdown.
If you have a doctor that can't explain things to you and communicate in a two way conversation, you might consider looking for another. Find out which cycle themselves. That will at least give you common ground to have a conversation. Make sure that you aren't the one that is not pulling your side of the conversation. I'm sure many doctors that would talk don't because most of their patients just want to get out of the office with a prescription.
If I don't cycle for a few days to maybe a week or more, I start feel blah and unhealthy. Like you, I start to do something that gets my HR up and I feel great. So that in itself is just a natural and common thing..... I think.
As for not reaching max HR..... How do you know what your max HR is. Formulas don't give you your max HR. They only estimate what it is for the average person of that age. And I think you alluded that you may have had a small heart attack or something. So your max HR may be well different than what is the average.
Did your cardiologist give you any restrictions for getting your HR up or exercise of any sort?