Any update on this? I'm in a very similar situation. Ryzen 9 5900X, RTX 3060Ti, ASRock fatal1ty b450 mini ITX, Corsair SF600 platinum. I can get it to post with my old Ryzen 5 1600, no other changes. Same issue when using my old 1060, or no GPU at all. My mobo came with BIOS v4.80, so flashing...