What’s your goal?
If you’re trying to hold endurance wattage because you’re trying to build up that ability, then keep erg on. If you’re trying to learn what it FEELS like to hold endurance wattage, turn erg off and see what a pain it becomes.
For me, holding endurance numbers is crazy hard. I want to dip into the very top of zone one because it’s a bit more comfortable or I’ll end up into the wind and start pushing zone three. To HOLD a true zone two endurance ride taxes my brain WAY more than my body. So, if that’s your goal, then turning off erg can be a way to get there, but if you’re after the body adaptations, keep erg on and just pedal away.
As for Zwift, forget it. I find any time I try to ride “easy” in zwift, someone sits on my wheel and testosterone takes over. What was supposed to be Z2 is now Z5 and I’m whooped way earlier than I should be. For me, Zwift is best for climbing simulation.