After going with Xiaomi devices for some years (with Gadgetbridge) I revived my trusty Pebble Time Steel only yesterday. This thing is 8 years old and doesn’t do heartrate but it still tracks my sleep more precise than any of Xiaomi’s devices ever did. Maybe you can find a device in good condition on eBay? Be sure to ask sellers for daylight pictures of the screen with settings menu or a bright watchface. If the battery is dying the screen looks like nacre cause it’s being lifted by the battery.