Hi
I’ve also just purchased a SB20 and just a few inputs from me…
I don’t have any issues pairing with Zwift. I’m using ANT+ and it pairs 99,9% of the time, it´s even located next to my Neo also running ANT+
I have experienced a few dropouts 1 - 1,5 hours into a ride, but it´s very rare.
I normally start and connect the Stages Link app first, of course it’s important that the trainer control is set to external, and then Zwift and it basically works.
I do have to pedal 5 - 10 seconds before I see the power on Zwift, but after that it is in perfect sync with the Stages app ( assuming you set Zwift to 1 sec. power updates ).
I’m not using ERG mode so no feedback on that.
Compared to my Neo I do feel the resistance control is quite laggy, but then again, the SB20 is very smooth and with the big flywheel it might just be how it is ?
I haven’t really had time to look closer at the config files, but if I remember correctly there is some hidden “trainer smoothing” setting somewhere that maybe could be altered.
I’ve requested Zwift for the gear display on the HUD in this forum Gear display for Stages SB20 and directly via email to support. You might want to vote on the forum thread and/or email Zwift with the same request 
RIDE ON 
BooX