I've recently been wondering if it would be technically feasible to make one of those more traditional metal watchbands, where the metal elements would be Li-Batteries. Perhaps too small though.

I think the tricky part would be the linkages where you'd need an electrical connection. Making that robust enough to be in the band but not too bulky would be a serious challenge. You'd also lose the big variety of bands and the sweet profit margins that they bring.

I don't know if there's been a regression since the series4, but my Apple watch easily lasts 30-36h (maybe I'm just too sedentary these days with WFH + occasional ride).

Even on busy weekends where I'm out all day with multiple "workouts" it usually lasts the day fine.