How to make a ground plane for relay PCB switching mains?
Definitely do not extend the ground plane into the mains area.

The relays are providing your safety isolation here. Draw a line (real or imaginary) across the middle of all the relays. Keep all low voltage tracking and components 'this' side of the line, keep all mains stuff 'that' side of the line. That will automatically ensure you maintain adequate clearances between the sides.

It looks like you have an unholy mix of low voltage and high voltage connections side by side in the same terminal block. Easy for channel identification. Perfect for lethal error. Have the control terminals and 24v one side. Have all the mains terminals the other side. Put the relays in the middle, keeping them apart. Line the control and output connectors up in the same order, if you want a nice visual channel identification.

On a board of this type, you do not need a ground plane at all. Far better to just track it.