By the way. I’m currently using the Residio RTH9585WF1004/U which actually works well. But I don’t think I can OH to it.
The VenStar looks promising. If what I have can’t be interfaced and I have to buy something… then going $200(+) for the VenStar isn’t a blocking issue. Not that I have $$ just laying around.
By the way. You seem quite Thermostat literate. 
I’m assuming the reason I can’t simply intercept and hack the WiFi/Zigbee etc for themostats is they require some kind of encryption or key?
I rewrote the code that drives HestiaPi and spent many hours trying to make the house comfortable without an AC. You pick up a lot under those circumstances.
Zigbee requires a Zigbee coordinator. But I believe you already have one (given the Sonof door contacts discussion on the other threads) so there is no reason you can’t use a Zigbee thermostat right now.
Zwave requires a Zwave controller but if you have one of those, again, there is no reason why you couldn’t use a Zwave thermostat right now.
Both are local only wireless protocols, no cloud involved and both are well supported in OH. But both are unique wireless protocols requiring a transceiver not typically found built into computers.
For WiFi, either the communications between the device and the API is going to be encrypted or that’s not a thermostat you want to be using in the first place as it’s insecure. Sometimes, rarely, you’ll find someone on GitHub who has managed to reverse engineer an encrypted protocol and posted a library that enables local control but that tends to be brittle as any change to the API could invalidate the library without notice. All the residio/honeywell home libraries I found back when I looked use the cloud API.
Looking at things, I think the VenStar will actually work well for me. And, although tinkering is fun, it’s an out of the box solution with an OH binding that I can easily integrated. A nice replacement for my Residio.
Zigbee thermostats supported by Zigbee2MQTT: | Zigbee2MQTT or | Zigbee2MQTT
Z-Wave thermostats: https://products.z-wavealliance.org/Search/Index?regionId=-1&searchText=thermostat (check the OpenSmartHouse Z-Wave Device Database to make sure the Z-Wave device is supported by the openHAB Z-Wave binding - but even if a Z-Wave device isn’t supported yet, it can be added to the database by the community).