futureslay

Thanks for the reply, Jorge.
 
I wasn't going to come back here, but I had to come back hat in hand.
 
So, I tried a new flash drive, used the USB creator with 6.12.8 to avoid any unfair advantages of newer versions. I setup the same static IP assignment in the tool and booted it with exactly the same behaviour. However, I then did something I suppose I should have earlier today and changed the static IP assignment by 1 digit. This solved my problems instantly, so I knew that it was nothing to do with my configuration and simply something to do with that IP address.
I booted my usual flash drive and verified the same by changing the IP assignment, and then used
 
arping -I br0 10.0.0.100  
getting a reply from my old motherboard's MAC address. Huh.
Turns out that despite not having any DHCP reservations setup for that IP, I still had the ghost of the old board hanging around. I rebooted all of my networking gear and flushed every cache I could, all to no avail. I even factory restored my router and reinstalled a known good backup of it.
 
Turns out that I have forgotten about the MAC/IP binding I added in the router a few years back after I realised my router didn't care much for DHCP reservations, but seemed to honour binding (can't be that good though since the new machine was still fighting its way through).

So, for anyone with a TP-Link router experiencing what appears to initially be an IP conflict, check your binding settings in your router config - mine's under "Security", nowhere near any of the DHCP stuff.
 
tl;dr - I forgot I bound the IP address for the server to its MAC. I've updated the MAC binding and all is well in the world.

Thanks again and cheers for reading the story.