I have smart thermostats, specifically the AVM Fritz! ones. They only require their own router (which, where I live, is a common thing to have) and use DECT to

I have smart thermostats, specifically the AVM Fritz! ones. They only require their own router (which, where I live, is a common thing to have) and use DECT to talk to the router (you can find DECT usb sticks that work on Linux, the protocol they talk isn't super complex). They run about 2-10 years on a single battery, depending on use. And they're fully offline, the router has an API I can control them from (HomeAssistant instantly picked up both thermostats and lets me control them), pretty much everything I want. AVM's smarthome story is miles ahead of Google/Apple/etc. Same for IKEA's smarthome stuff. If you need offline capable things, use those.