Further update. I contacted a store and agreed I can order cards and test them at home, then return them if they don’t work and order another model.
If you are in the UK I highly recommend OCUK as explained here, where I have compiled a list of cards that are reported working, along with cards reported as NOT working.
In short: my configuration was 100% correct. I found two cards [EDIT: two 6000 series cards, plus my older RX550 and RX480 with vendor-reset patch] that “just work” as opposed to my Gigabyte Aorus Elite 6700XT which refused to work no matter what.
Clearly there’s something vendor/board-specific that causes this. Please refer to my Reddit post for a list of reported working/non-working cards before you purchase.
I personally highly recommend OCUK for a no-fuss experience if you are in the UK.