So I've dusted off this project and made some headway. I would appreciate some feedback on what is good, bad and unnecessary. The traces in the centre consist of a common 24V source and then each relay coil has a lowside driver (on a separate board, connected via the header on the right).
All these traces are surrounded by a copper pour top and bottom which is connected to mains earth. The idea is that should any mains voltages track over, it would touch mains earth before any isolated low voltage circuitry. This should then trip the RCD breaker.
Each relay has an isolation slot, though I wonder if they are really going to do much in this case. I can't make them any wider due to structural reasons on the PCB, and they only slightly increase the creepage distance. For reference, on one of the pictures I have added a couple of measurements on the left hand side, ~10.3mm and ~13.3mm which seem to be very generous clearances according to what I've read.