I have a good experience with Huawei Band. Huawei doesn't even require to pair the watch with an official app! So I never connected it to the Internet or installed their app, paired directly with gadgetbridge from the first day.
Amazfit required a one time registration to get a one time code to unlock things, but I used a throwaway email id, and then deleted their app immediately. After that point the watch only syncs with gadgetbridge. The best part is that the watch has no internet capabilities, and gadgetbridge itself doesn't have internet permissions either so you know that absolutely no data can leave your device