Intervals.icu has a specific stat for this “Power / HR in Z2” that you can plot over time to see how your base is improving. It only counts time spent in HR Z2. There is also a slider under the Intervals.icu decoupling charts to exclude warmup and cooldown time which also applies to related stats like “Power/HR Z2”.
This is the best decoupling chart IMO:
It shows HR as a percentage of HR reserve (max HR - resting HR) overlapped with 60s power as a percentage of eFTP. It adjusts the scales so the traces start out overlapped so as decoupling kicks in they start diverging (more bpm required to get the same watts). It saw this in a Dr Stephen Seiler video (link).