> Doctors will often recommend exercise, but I find that these days even moderately strenuous exercise like riding a bicycle destroys my sleep quality for several days. There's something about it that appears to be too physiologically stressing, even though ten years ago I was a happy as a regular gymgoer.

I had something similar like this. I think I was able to fix it. The theory is that your sleep is still poor, even though you sleep through the night. This is causing high cortisol levels during day time and higher resting heart rate. This is elevated further after doing moderate exrercise and takes a long time to get back to normal as your sleep isnt adequate. If your heart rate doesnt go down enough, then your sleep quality gets destroyed.

The solution, for me and I am guessing for you, is this: Stop the cycleing. First fix sleep. Track it using Wellue O2 Ring. If the scores are not good, the reconfigure CPAP - use sleepapnea reddit for inputs. Once sleep is sorted as per O2 Ring, then it might take a few months for you to recover. After that you can restart moderate exercise and things should be fine.