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.
I had a good experience with Daikin split systems, but it’s really going to depend on your region and what’s available in your area, or what you’ve already got. If you’re looking for inspiration on what works well, check the Home Assistant forums.