Looking at the workout definition for Xert - Login Xert Fitness Test for Breakthrough Version 3 there are no slope mode intervals (despite the #mixedmode hashtag in the description) like the other fitness tests for breakthrough workouts, so it appears you can do the entire workout in ERG mode. I don’t know if the intention is to still be able to change gears during this workout.
There is some additional discussion in the following thread about virtual shifting and single cog trainers: Zwift Cog and Click (and Ride) support?
Not having an on-bike / outdoor power meter and also using single cog trainer, I’ve found doing virtual free rides on other platforms useful for keeping my fitness signature honest. E.g. MyWhoosh, TrainingPeaks has free virtual Tuesdays, and Rouvy has 20km free per month; pick a ~20km route with good climbing or sprint segments and give it a full send. MyWhoosh and TP would need a middleman like BikeControl to use virtual shifting, and Rouvy natively supports Zwift Click gadgets. TP also just released an update where they announced implementation of the OpenBikeControl protocol, which is good progress toward hopefully standardizing virtual shifting hardware communication.
I’ll have to give the V3 breakthrough workout a few tries though and see how it feels and what happens.