I have the same issue, ERG doesn’t work at all. I’m using Zwift on my MacBook Pro 15 inch running on the latest version of Big Sur. I’m using a Garmin Watch to broadcast my heart rate.
I tried to turn off Bluetooth on my iPhone where I’m using the compagnion app but same issue so I don’t think it’s related to Garmin Connect. I still tried to uninstall the Garmin Connect App but nothing changed. I turned off my Garmin watch, turned off every Bluetooth device I have other then the trainer and my laptop. I calibrate the trainer multiple time. Still had the issue. Restarting my laptop seems to improve and reduce the delay.
I can confirm it’s a sync issue, Zwift has a huge delay when it try to change the power.
Running my workout on the Zwift iPhone App seems to be better.
I’m available to diagnostic the issue, I’m in tech so can help debugging if the Zwift team need someone.
The issue is annoying, I just bought this trainer and never had issue before when I was using the Tacx Neo 2. I really think it’s a issue between Zwift and Saris H3.
Also if you try to calibrate the trainer via Zwift, it doesn’t work with the latest Firewire in the trainer.
Willing to help if I can.