I got a Google Nest E thermostat off eBay from a charity shop for £12 and wired it in to where my old dumb dial thermostat was.
My ZigBee thermostats are just my ZigBee motion and door sensors that also have a temperature element.
I turned off the Nest smarts in Nest, and had HA come up with average temperatures for the whole house using the ZigBee things, then recreates the smarts in HA.
The Nest E smarts stopped working 6 months later but the heat link still worked, so I bought another off eBay for £20 and paired that.
I am gonna just get some ZigBee temp sensors at some point, but this works well enough for now.