Johnny, each device treats ERG mode a bit differently. My understanding is that in ERG mode it will pick the midway point of your custom Zone. If your Zone 2 is 180-210, it will set on 195 in ERG mode.
My recommendation is to perform the workouts in free mode and learn to discipline yourself to stay at a particular intensity. This is how you’ll have to pace yourself outside or in a race, yes? There is certainly a place for ERG mode, but it should be an exception.
David