Garmin Fenix 6 (or equivalent) and above work fine for sleep tracking with gadgetbridge and endurain, and are pretty cheap secondhand. Battery life and build are stellar (and seeing as you charge it at most weekly unless hammering GPS and Music, 500 charge cycles is a decade, batteries are replaceable too, screws, but I'd be leery of diving with it afterwards) and you get all the smartwatch / sportswatch stuff. It's sleep, as long as it's +- 5 min, it's good enough to work with, doesn't need to be the best on the market.