You're looking at the right areas but I think you're drawing some of the wrong conclusions. Higher rpms uses slow twitch muscles not fast twitch (I know it sounds backwards). And what you said about lower rpm's being better for endurance is backwards too as a result. You said that higher rpm's raises your heart rate more which is why it's worse. And it's true the reason it raises your heartrate is because it pulls more from your cardiovascular system than your muscular system. But why would you want that? Because the cardiovascular system has far far far greater recovery rates. It's why a pro can attack up a hill at max effort, then fall back into the peloton for an hour at zone 2 and their legs are ready to go again for another attack. While do it at a low rpm and you're probably done for the day and maybe even the next day. You're realising your cardio is your weak spot so you want to reduce it in your riding, but I think that's a mistake especially as you get older and you want to keep your cardiovascular system in as top shape as it can be. I think instead you need to forget about speed, especially now as you have a power meter. Train to your power and heart rate zones, and use your efficiency factor (normalised power divided by average heart rate) as a measure of progression instead speed. Now I'm not insisting that you need to force yourself to use a high cadence, but you don't want to be forcing yourself to use a low cadence either.
Also storing wine in your conservatory is possibly the worst possible place you could have it lol.