The Z2 work is really beneficial and hope you’re having a good experience / outcome to that.
I saw that TD has this feature where you can scale / put an offset of Power at specific HR (much like drawing a graph and connecting a best fit line). Does this serve the purpose you’re looking for?
The “Set Target HR and have trainer adjust automatically” is not a new thing, the complexity is actually greater cos every human is different and thus the HR response but it is definitely easier to control than having say - using cadence to control Power Targets.
You could use Garmin device to set the HR zones and have it “beep” when you’re out of the targeted zone (that’ what I do for my Z2 running and just slow down or walk when it beeps)
I think TD can also do the Target HR workout thing (if not you can create one for yourself within garmin’s ecosystem) and send it to Garmin.
Alternatively, you can also use the workout builder in intervals.icu (https://forum.intervals.icu/t/building-workout-using-zone-number-instead-of-percentage-range/)
It all depends on which ecosystem you’re most comfortable with. Having said that, I do understand that these are not what you’re looking for specifically. YOu want the trainer to be controlled by your HR.