Quick update, as well as a warning for anyone in a similar position in regards to the Windows license.
The GPU itself is still struggling at boot, what doesn't help is that UEFI keeps re-enabling settings and the UI not actually reflecting the settings e.g. I enable Resize Bar, the GUI says it's enabled but because of the constant reboots it's re-enabling CSM without warning me which also disables Resize Bar.
I did manage to install the driver, which from what I've read should have also updated the firmware, the latter of which I was hoping would fix this.
I'm just going to wait now for the other card I ordered.
As for the windows license.....turns out it got transfered to the new machine as I temporarily needed Windows to do said firmware update.
I don't know how it happened, either the fresh Rufus made Windows On-The-Go USB drive also transferred the license (which would be news to me) or MS used the CPU as the basis for the activation rather than a combination of hardware like I'd expect.
If the latter is true, it makes me wonder if any second hand CPUs also come with a 'free' Windows license?
I ended up logging into the install using my MS account and making sure it's linked to my account, so hopefully in the future I will be able to re-use that license.