@ODY-GB
Yeah, both, server and client should use the same default gateway to communicate properly.

The traffic seems to stop dead at this point. I'm not sure if it's because the pfSense isn't able to match up the DHCP OFFER with the original request it received as it isn't on the expected interface, or if I do need to put a firewall rule in place here.

The respond from the DHCP server never reaches the client. So the client continuous sending requests as the packet capture on the guest wifi shows.