I re-made this video with a tighter edit — less boring, more to the point. Should be much easier to follow now:
This covers the custom PCB that finally solved the wiring mess and made the retrofit a lot cleaner. Hopefully this version is more enjoyable to watch!
The youtube comments pointed me to a issue relating to the wiring of the second output channel of the Photorelay ... so I have to go fix that with some bodge wires
As always, happy to share the design files if anyone wants them.