that’s another great idea: I could use also a Xiaomi door sensor (see below). PArdon my ignorance, but when the external doorbell is pressed, in the internal unit you think should have two pins and receive a relay-like signal (close on press or open on press)?
The above would solve the size problem (the xiaomi are very small, even smaller then the fibaro) and the power supply (they are battery operated and last easily a year)