Z1/Z2 Improvement Target: “Cardio” or Type 1 Fibers?

I’ve devoured TNA and have been going strong for a few months reversing over a decade of ADS and building an aerobic base. For Z1/Z2 workouts, am I right to focus on local muscle fatigue over more objective (though variable at this stage) breathing and HR metrics?

I consider developing the aerobic system as developing ample Type I muscle fibers with the lungs and heart as secondary support mechanisms. Am I missing a physiological component that would justify keeping heart and breathing rates up even while my calves, for example, start dumping lactic acid? My understanding right now is that it’s bad to feel any sort of burn on Z1/Z2 days and that unusually low heart rates are good if it keeps breathing and, more importantly, the anaerobic burn down. Conversely, running downhill gets my HR above the HR marker for AeT, but it’s okay as long as local muscle fatigue and breathing rates still indicate sub-aerobic work levels.

Maybe I’m overthinking it, but I’ve been really impressed with this system so far and want to make sure I understand its intent correctly. Thanks for all the resources!