Get an ESP32, a temperature sensor, and 4x relay board and build your own with esphome!

If you pull the instructions for your thermostat, the wiring guide should tell you what each wire is for (because you can't trust wire colors). From there it's just wiring up the relays properly, getting the config built in esphome, and setting up a generic thermostat.

It sounds kinda daunting, but it's really not super complex. The only gotchas too look out for are any of the relays that can't be on when another relay is on. There's a way to prevent that in esphome. I'm sure someone has made a guide on it by now. I would have made one if I had gotten my enclosure figured out before my 3D printer took a hiatus.