I have found several useful projects implementing OpenTherm communications the inter-pipes:

• Schelte Bron's OTGW project in which most schematics seem to be based including mine
• Garilov: https://github.com/gavrilov-i/OpenTherm https://habr.com/en/post/251539/ (in Russian)
• Ihor Melnyk: made a controller and also an Arduino library
• Jiří Praus: also made an Arduino library and interface including a very nice PCB.

I am basing my code on the last one, Jiří Praus' library is more "manual" and more "C" style but it seems to be the best starting point.

My code currently loops through 7 message types:

• Read STATUS
• Read SLAVE_CONFIG
• Read MODULATION_LEVEL
• Read CH_WATER_PRESSURE
• Read BOILER_FLOW_WATER_TEMP
• Read MAX_CH_SETPOINT
• Write CH_SETPOINT

And then repeats, reporting the current status. It also accepts a couple commands via the UART.

The code is on github.