

Not to sound stupid, but it really depends on how smart you want the watch to be. From connectionless firmware device to fully-featured Android. +1 for gadget bridge either way.
I have a Fossil Hybrid, that combines physical hands with a 2-color e-ink display. It can’t do apps, but it has standalone timers, notifications, media control, pulse/oxygen and step counter. I personally don’t need more. It’s cloudless and lasts a week.
If you need full Android/WearOS check AsteroidOS and specific ROMs. Hardware tends to be on the older side here.
The only thing that’s hard to do is sleep tracking. That tends to rely on proprietary algorithms and cloud compute a lot.
In reality they do help superficially, but they very much inflate their numbers on a shiny dashboard, showing you how much they’re helping. All while only hitting a small fraction of databrokers.
I also think, that as a subscription solution to a problem, they could turn into the online version of turbotax any second now. Lobbying for harder self-optouts so that their service stays relevant.