Gas hot water heaters tend to be a lot more efficient than gas cooktops. The flat bottom of most pots and pans means a lot of heat will escape up the sides, but hot water heaters can be designed with this in mind. On pots, I have one that basically has heatsink fins on the bottom to better capture the gas heat, but this is far from typical.
I personally use the original amazfit bip. Cannot be newthough, I sourced mine from aliexpress, and even there they are ex-demo and stuff. Heart rate sensor isn’t amazing but I don’t know if that’s maybe my hairy arm. The main reason I got it, can be used with gadget bridge, for best privacy
Edit: worth noting that I had to flash it with English firmware, which did mean I had to use the official app originally