As briefly mentioned before, I need a way to take the CNC4PC C10 breakout board's output signal (5V@24μA) and switch higher currents and voltages. After the 4 pairs of step & direction signals, the C10 board still has another 4 outputs (spindle, coolant, etc). CNC4PC do sell the C15 (a nice opto-isolated dual relay board). I should just buy two of these (they're quite reasonably priced), but this is about the journey, not the destination.
In pursuing modularisation I think that opto-isolation belongs on separate modules, so my quad relay board won't have it. All that's required is a few transistors to step up the current, some relays capable of 240V@10A, LEDs for diagnostics and supporting passives (current limiting resistors and fly-back diodes).
With the new X axis coupling, the board drilled and milled without losing position. The only issue was the double-sided tape holding the board down gave up during the final board shape routing (hence the lack of squareness). The isolation ran at 60mm/min and took about 30 minutes to run (I really need to find a way to get 30,000rpm instead of 1200rpm).
To make it easier to place the components, I printed the overlay on paper and stuck it on top of the PCB (using a needle to pierce the lead holes). The Ø1.2mm pads on the diodes & resistors were a challenge to solder (it was very easy to bridge across the milled 0.2mm isolation channels). In the future, for manually soldered boards, I should make both the pads and the channels larger (the board design didn't need such small pads - Ø1.5mm should be fine).
If you're interested in the tool-chain: Altium Designer for Schematic & PCB layout, CopperCAM for Gerber to GCode, Mach3 for Milling.
Milled & Soldered PCB
The top overlay and components