Doing it the CAN ways indeed seems most appropriate and more scalable across different ECUs, though I am not sure if adding CAN signals to the ECU is that much easier than what I did
It is. Even on much newer ECU's.
You're going to have to do a lot of reversing either way, but putting something extra on CAN is a much nicer solution.
The only exception are the ECU's that have already native flex fuel input support. Simos 18, and MED17.1.1 on 4.0TT come to mind.
