Personally I don't mind initial registration, so long as they don't continue to get data, so I'm not sure. There is some development for Garmin connectivity that I'm hoping comes to fruition.
I did like the old amazfit bip and that could be used without registration using the Notify for Amazfit & Zepp app, from what I can remember. I never used it with Gadgetbridge though, and it didn't last for very long before dying (cheap build quality). It isn't much of a fitness tracker either.
No headphone jack is one potential catch