I have finally finished firmware for the TeHyBug CO2 FreshAirMakesSense Boards. The firmware has the most needed features for smart home integration like a webserver that serves sensor data in JSON format and autodiscovery mode for HomeAssistant. It also supports OLED displays and some more features. For the complete feature list and description please visit the GitHub repo where I have published the firmware.
https://github.com/gumslone/tehybug/tree/master/tehybug_co2_firmware