I suggest you allow users to pair directly to the Garmin 520. Take the data collected there and display it in Zwift app. Pairing to a variety of sensors seems like a waste of time when it is already available in the 520.
i have been trying since November to pair my Garmin ANT sensors to my iPad Pro and iPhone 6 without any luck. So far I have ridden 0 miles on zwift rhis winter.
Please consider making it easier to connect. Note I will not be buying a fancy new trainer as the I have works just fine. In any case I have invested over $2000.00 on studded tires and wheels for my various bikes. Riding outside is always more fun than zwift.