Quick update now before I go to bed: the red bits are now on Patches - assembly by mallet is so satisfying! - and I had to redo the Y bed to centralise the drive shaft more. There is also one more 608 to further constrain the left-hand Y rail. The right hand one remains relatively lightly guided.
I've got the switches mechanically located and functioning, though the Y bed motor bracket needs a protrusion on it to strategically poke the switch with. I just free-welded some PLA into place by hand and I'll update the files later.
If I run in to an unsolvable problem with even-sided drive on the bed, I can simply duplicate the Z axis drive components and run two drive rods. It's tempting but I want to go really minimal for this build.
The mounting bracket for the stepper drivers needs designing, but that'll be an integral component of the Arduino mounting backboard. There will also be a patch-pad holder for discrete components. This one is designed not to have a custom PCB, and to be ultra accessible. Replacing or substituting components will be a relatively simple process with no soldering. I hope.