I pulled the C axis nano off the Eleksmaker board and moved it to the Funduino shield, eliminating one controller. The Y axis driver now drives both Y motors via a splitter - eleksmaker style. The 4th driver on the protoneer is now dedicated to C, having been modified by removing its ENA pin and resoldering it in reverse so it can be patched back to the C nano ENA pin 8.
This also allows grbl to control the ENA on the C nano so it doesn't cook by being on all the time.
I may still strip the C axis nano's grbl code of YZ and add more function for sensors and solenoids etc. We'll see.