Since a while without updates regarding the project, some news:
I decided to use Go instead of C++ for faster development and keep a reasonable lightweight. Main OS will be DietPi with i3, way more lighter than Raspian distros, even if need some more configuration, I am happy with this choice
The case and the connections need improvements, I want to merge most wires in a single socket.
There are some issues with the power feeding, powering the monitors outside the RPi does seem to solve that, especially when all the peripherals are connected, may I will switch to a single screen.