Best place to start would be to look at the thermostat hardware you’ve currently got, and start searching online if anyone has integrated it into Home Assistant.

I’ve lived at a few houses now with Home Assistant. In all of them I was able to integrate my HVAC and automate it, but some brands and hardware are definitely easier than others.

I think the most extreme of them required a custom esphome device connected to its PCB to talk to Home Assistant, and another required me to write my own custom component.

Hardware and brands make a huge difference, but sometimes you’re stuck with what you’ve got.