Now you’re getting into ventilatory thresholds, which are totally different from LT (LT1, LT2, etc.). But using VT1, below which your breathing is very controlled, you can talk relatively normally, as a guide for RPE is a good move. I do it all the time. Conversation-paced riding and running is the epitome of zone 2 (as long as it’s above recovery pace, that is… you should notice some difference from resting to working). You can get a sentence out without gasping, but you need to take a break from talking to breathe far more often than standing around chit-chatting…
Most people aren’t going to see a noticable difference in their breathing when HR goes up a few beats at the end of a long ride like that. Your body is getting plenty of oxygen into the lungs, but HR is going up to increase the supply of oxygen to the working muscles as they deplete glycogen stores and increase aerobic demand. So your ability to take in air (and uptake oxygen to the blood) and remove CO2 from the blood isn’t the limiting factor, thus your breathing rate doesn’t need to increase.
Again, that’s a sloppy description, but close enough for coaching work.
There are discreet ventilatory thresholds where if you learn to monitor your breathing, you can get a really good idea of your exertion. Like HR, it’s time-late, but for steady state it really works well.