So, after working on my multicolour FDM project, I decided to just aimlessly scroll AliExpress to see if there were any new and interesting products that I've never seen before, and I found a very low cost and low footprint motherboard, the Bumblebee from Macrobase:

This is the kind of controller board I've been looking for, since the SecSavr Suspense uses so many stepper motors. It's much smaller (90 x 125mm) and cheaper than the MKS Monster, and could mean that I could opt for the £13.42 cheaper M4P instead of an M8P. I probably won't though, since the M8P is laid out more orderly. I've got to take wire management into account when making a motherboard decision.

The M4P seems to have one more physical USB port (and not a header), but since I'm planning to have a lot of USB cameras dotted about like it's a mini-factory, I'll be using USB extension hubs anyway. 

Anyway, 2 of those Bumblebee boards costs about the same price as many singular 8-stepper motherboards, so I'm just going to see it as a split-in-half 16 extruder motherboard and embrace the FREEDOM of going with as many stepper drivers as needed. I might give each stepper it's OWN stepper driver, putting the ones I'm most likely to need to tweak current values on the M8P and all the rests on 2+ Bumblebees.

Oh right, I should mention that I'm going to be switching to 3 or 4 Z axis motors. Manual bed levelling should be a thing of the past, especially if aimed at consumers (though I'd imagine only prosumers would actually consider spending >£1500 on a Do It Yourself build).