Thanks for all the replies guys, I've decided to send the GPU to a repair company here in the UK, CableMod did recommend Krisfix in Germany but would probably take twice as long and not too comfortable sending my GPU on a tour of Europe tbh lol.
The UK company have said they've repaired a lot of these already, had over 20 4090's in last month alone, all using a wide variety of cables and all failed on the same row of bottom pins melting, I don't think this issue is as rare as Nvidia would like people to believe.
I don't hold Asus accountable, they are working on slim profit margins and they are within their rights to void warranty with 3rd party cables much like all the other companies do. It's completely shocking from Nvidia, every single one of these things should be recalled and fixed.
When a fairly small GPU repair company says "Oh another one" over the phone when you tell them the problem, then it's not exactly a rare issue is it.
If Nvidia are still using this connector on the 50 series cards then I'll be using AMD from now on