Yeah, if you’re doing 3 min VO2max workouts, don’t wait on your HR. For 3 min intervals, it takes several intervals before my HR even gets to/above threshold HR, while I’m 115%+ above FTP.
I’ve been using HR for about 9 years (and started using power a few years ago, but inside only). You should do the Friel 20min test to get your LTHR (and do it periodically until you get it dialed in). Bear in mind, when/if you do start using power, that the lower Friel HR zones (% of LTHR) don’t match up with the lower Coggan power zones, so I had to switch my HR zones to Coggan’s so I wouldn’t lose my mind.
You should check out the TR thread about how to do VO2max workouts. The method many tout doesn’t require you to try to hit/hold a power or HR number (though I usually start with getting up to a wattage I know I can hold for the prescribed interval and go from there, based on how I feel). Kurt’s posts lay it out pretty well, though it seems some may be missing now.
Here: What would a custom VO2 max progression look like? - Training - TrainerRoad