Withings Scanwatch is cross-platform. No need for a branded device to use it, does all the usual tracking stuff plus afib detection. One of the reasons I chose it was that it looks like a watch and not a toy. Also, depending on how you use it, it can go for days between charges. Most of the similar “brand locked” devices seem to only go 1 or 2 days max.
I have an older Withings watch and will buy another once I need to. Even bought a set for my parents. I really dont need a tiny phone strapped to my wrist, I have an actually usable one right in my pocket. The battery lasts weeks, the heartrate and sleep tracking stuff works, I can keep the phone on silent and still get notifications… it really is the perfect smart watch as far as I’m concerned.
