the US version of the Schneider wiser air appears to have ZigBee, according to their spec sheet, and supports forced air heating. Not sure how accessible its API is for building a binding. It also appears to be an almost direct back-plate compatible with the Nest and doesn’t look too bad. Unfortunately, I live in Australia, so not readily available here, and I don’t have ZigBee deployed.
I am, like everyone else, very annoyed with Google for this change in service, as I love the look and functionality of my Nest Thermostat. As for DYI, I’ve been playing with Arduino, WiFi, MQTT and OH to control curtain motors, and I can see how this might work with building a thermostat. Getting it to not look like DYI is the trick. There are a number of sensor and screen options for Arduino, and building a small power circuit to run off the 24V supplied by HVAC wouldn’t be difficult - I had to build an interface circuit for my Nest anyway as Australian heaters do not conform to the same standards.
Best of luck,