KuristZero

Thanks for the suggestions folks, I think I found it - hidden setting in the router. More info below!

Bonding
Good catch on the network bonding. I've turned that off for now, but this didn't resolve the issue when returning to the old IP (again, router side anyway).
Any particular reason to have this on or off? I've not touched Unraid's networking settings much outside of Cloudflare tunnels etc

Network.cfg
I did wipe the network.cfg as mentioned above, but was unaware of network-extra.cfg - is this meant to be in the config folder, as it's not appearing in mine? Is that a standard file or a potential extra that shows up?

Router side
Router has gone through several reboots too, and I removed the old static IP and MAC address from every setting I set them in while the server was powered down to prevent any accidental reservations.

HOWEVER the problem was an automatic setting that TP Link had auto-activated after I updated the routers firmware a year back (security issue in the code base, so needed the update). For anyone facing the same issue with a TP Link router:
Head to Advanced Settings > Security > ARP Binding (or IP & MAC binding depending on your router).

This was now turned on by default after the security update, probably as a "security feature" and had all the reserved IP addresses added from the DHCP reservation list.

Once I removed the old IP and MAC combo, we were back online!

Thanks so much for the suggestions and guidance folks!