Just like with the 8-stepper Bumblebee, I was once again looking around for motherboards because of my Coaxial Hotend project and came across this:

This Mellow Fly D7 v1.0 board is low cost, compact and has all the things that I actually need (UART-enabled stepper ports) and trims out many of the things that I don't (since they'd be on the Manta M8P V2.0). 

The most notable ommision are EXPn, meaning that I'd only be able to use 1 motor expander (on the M8P). That should be fine, as with 2 Fly D7s and 1 BTT EXP MOT, the printer will have 25 steppers to work with.

It might be useful that CANBUS devices can be connected directly to the board without a separate transceiver.