https://apps.garmin.com/en-US/apps/95fc3b69-6c3c-4618-a2de-f68ffca35360
This certainly isn’t perfect but you can set a custom floor that you have to exceed (just can’t give a ceiling).
Thread here: Announcing "Endurance in Zone" for Garmin devices
@torhovland is the creator
I have not tried this but could you use Garmin’s native Time In Zone and customize your power zones?
Default:
Sweetspot specific:
