Well, I am typing from the phone right now, but the short version of how I sorted my problem was to flash the 828 bios version from the "secret" webpage as opposed to the one that is shared on the wiki.
They are supposed to be the same but... Using the other one I managed to ignore the guides and completely disable internal GPU and all worked.
Thanks for the reply! Any chance I could get the link you used to get the version of the 828 firmware that worked for you?