Honestly I think people over complicate things a lot. To be fair though that’s a lot of fun.
Personally for z2/endurance stuff I would ride based on RPE.
I have experimented with lactate, checked out dfa alpha-1 using a polar h10, and monitored respiration rate. All of them get me in the ballpark but I have found it’s easiest to just ride at 3-4 maybe occasionally 5/10 RPE and monitor decoupling over time.