Just a toggle switch in the setting with a space for max heart rate. Since %HR is a very common metric for measuring effort.
Agree, I always use %max when I do workouts.
Just my personal opinion, but…
I would rather keep the interface a simple as possible. Most of us have Powermeters that display those metrics without the need to add them to the interface.
Once again, just my 2c worth.
I’d like to see this as well - I do not run my Garmin at the same time while using Zwift.