Are you open to building your own? I built my thermostat a few years ago out of an ESP32, a temperature sensor, and a few relays, and I've been really happy with it. It's reliable and easy to integrate into my automation workflows and collect data from.

I am and could, but can't make the time for the inevitable support issues that pop up. If there's a COTS system available, I'd rather spend my time in the mountains :).

The upside of a home-built system is truly serious logging, something that seems to be a nonexistent priority for any commercial option I've yet encountered.

> the inevitable support issues that pop up

They may not be as inevitable as you think. I would consider my thermostat to be pretty casually slapped together with no professionalism whatsoever, and yet I've never once had it just randomly stop working in the 4 years I've been using it. I have broken it by my own actions while hacking on it of course, but if I don't want to spend time fixing it, then I just don't mess with it in the first place and it keeps chugging along.

I do keep my old "normal" thermostat around as a backup, and I've reinstalled it once when I had an HVAC technician over because I assumed he'd need to use the thermostat at some point and I didn't want to freak him out. I've never needed to use it otherwise, but it's always there to keep me from freezing in case my custom one randomly explodes and I don't have the time or desire to fix it.

If you do end up wanting to build your own, I documented a lot of my original build [1]. It's more of a journal of figuring out the design and build process rather than a guide, but I think it does a decent job of showcasing how straightforward it is to build one, and there's a schematic and link to the firmware source code at the bottom.

[1] https://blog.benhaney.com/2019/03/26/building-a-thermostat

Very cool. Thanks!