I'm using a Xiaomi Smart Band 8 - see https://discuss.grapheneos.org/d/35993-step-counting-app
Works with Magic Earth (now deinstalled), Organic Maps, CoMaps, OSMand+, as well as with the German Tiny Weather app (from DWD). It also receives notifications from your phone in case you'd want this.
As I've read, newer devices (Xiaomi Smart Band 9/10 and their Pro cousins) have better sensors, and so their heart-rate data should be more exact. There's Rob explaining this as part of his job on YT: https://www.youtube.com/@TheQuantifiedScientist/videos