Not sure if you had a look at my system:
https://forums.kinograph.cc/t/my-scanner-arduino-pi/1949/2
I use 2x uno & cnc shields. Main reason is simplicity, one uno looks after the takeup and back tension and is independant. The other drives the film advance motor and takes care of the reference pulse from the encoder. I need that pulse to be an interrupt but if I put all three motors on the same board I cannot use the interrupt pins 2 or 3 as they are used for the stepper drivers.
The cnc shield has several analogue and digital lines available for other use, check actual connections with a meter as not all shields are wired the same… been there. Generate your own schematic right from the start to avoid head scratching later. If you need to send more data then I2C is available on two unused pins and can be connected to the pi via a level changer.
Cheers