Well, I installed windows using GPT.
It still doesn't let me boot with Above 4G Decoding set to Enabled.
How come it's giving me a blank screen again?
Diskpart shows my drive is in fact in GPT.
Pick out motherdboard battery for 20seconds or so, restart windows without any gpu, shutdown.
Setup bios as described before. Restart.
Then install all gpus again, and finally turn on rig.