I use a Xiaomi Smart Band 7 and pair it with Gadgetbridge, and it works fine for my purposes, which is HR-monitoring during the day, sleep and workout sessions. I rarely interface with the watch itself (which is by design), so if you want more functionality out of your watch, then this might be a little on the light side feature wise. I tend to keep Bluetooth off, so I connect to it maybe once a day to sync data with Gadgetbridge, which I again export for analysis. A bit clunky to connect - I have to search for it first in the Gadgetbridge app, and only when it has found it can I attempt to reconnect. Maybe this is easily fixable, but I have not bothered to do it because I only sync once a day.

You do need to obtain a key first though, which requires a login to the Xiaomi servers. I used a throwaway e-mail for the registration. Gadgetbridge has no access to the internet.