Definitely Zigbee or Z-wave instead of wifi. Once you start to build out more devices you will be glad to not have gone down the wifi route.

I went with Zigbee and have Sonoff SNZB-02 temperature and humidity sensors in each room that have been absolutely flawless since the day I installed them, and they last about two years on a single battery!

I would obviously recommend them but I don’t have any experience of other thermostats or of Z-wave.

I did also install smart Zigbee TRVs on each radiator but I don’t rely on the internal thermostats of them as they don’t accurately represents the overall room temperature.

I’d recommend checking out Gadgetbridge. It is an open source app, with no network permission requirements, that replaces the OEM app for a large number of watches. I’m running an Amazfit GTS 4 with it and it has been almost flawless.