The interface is complete, I wanted to use a generic STM32 (bluepill) micro but also had an ESP8266EX, wemos D1 mini (clone)  and an OpenTherm library that was already working with that. I did not wanted to fiddle setting the 3 required hardware timers that the library is using so that settled it.

The circuit is quite clever, based on the schematics for the OTGW, I only changed some resistor values to match the optos that I had lying around (4N28) and the fact that this micro GPIO is 3.3 volts, and not 5 volts like the PIC on the original. The big RJ connector is not connected, I might use it for power if I go the WiFi route instead of serial. I'm not sure if I trust the heating to my WiFi router, thought. Maybe not.

Here is an excellent description of this circuits's workings:
https://electronics.stackexchange.com/questions/466720/how-does-this-circuit-work-opentherm