After a reeeeally long time I managed to get the firmware working with my custom PCB. PCB design has some flaws so there is still a potential for revision 2 of the PCB. I added an option to erase bonded devices and start pairing new one - use a button on PCB or hold mute button on the steering wheel for >3 seconds. Made a lot of tweaks to internal workings of the firmware and device behavior. Additional 10k resistor and 2.2u capacitor is needed as a power-on-reset on MCUs reset line to start properly, without it device is working but is not receiving CAN frames (maybe erratic initialization of MCP2515 on power-up?). Got it in my car conneted to OBD2 port and really happy with how it works. Spotify starts automatically on my phone after first button press to change song - nice not-intended feature :D

I'm happy with how it turned out, feel free to leave any comments either here or on my github. Maybe I will test other GM/Opel cars and update compatibility ;)