Rx 580 - can't get it to work

Hi all.

I'm stuck, need help. I have a Hack that's been working beyond smooth for years. With Mojave though I decided to upgrade my GPU from a GTX660ti to an RX580, since there are some nasty graphics glitches with the 660 on Mojave.

I had already upgraded to Mojave with my Nvidia card, and swapped out my AMD card.

On boot, some weirdness happens: I cannot go to Bios no matter what, and to get to Clover I need to trigger the "select boot disk" menu.

If I do that I get to clover, and can select my install, but immediately after the apple logo, the screen goes to Shit and I get nothing but static.

I really thought it would have gone smoother... I know I'm doing something wrong but I can't find what for my life! This smooth build didn't help: I never really had much learning to do to be honest.

I'm on a Gigabyte Z77X-UP5-th mobo with a Sapphire RX580.

ANY help, even proper google searches will be appreciated!