I had heard good things for Ecobee, but at some point last year they shut down their developer registrations. Still 100% accessible through the HomeKit integration, but it didn't feel like a good move for them.

I ended up going with a Meross MTS300 to replace my Nest. It feels like the local-only smart thermostat market is pretty dry, so I gave them a shot. There's some 3rd party Home Assistant integration for it, which I actually saw a Meross dev offering up some internal docs in an issue so they could build better support. I thought was pretty chill of them. A much better attitude than "oh we don't support this platform officially, cease building for it". Supposedly it can operate entirely separately from their infra with some tweaks, but I've unfortunately ran out of weekend. Even without those tweaks it can operate without internet over Matter

Wouldn't recommend it personally yet until I have more time to find the quirks about it. In any case it's been significantly more reliable than my Nest ever was

You should be thankful for the privilege

It still functions as a thermostat. You simply can’t access the smart features like presence sensing (to automatically turn your system off when you’re not home) or cloud control, (so you can’t adjust the temperature from your phone anymore). But it’ll still continue to function as a dumb thermostat. You set the temperature at the unit, and it holds said temperature until you change it.