Can't see BIOS (no input) after switching from 1050 Ti to RX 580

Hey everyone, how is it going? I need your help.

After two years running a beautiful stable build on High Sierra (i7 4790K + Gigabyte GA-B85m-D3PH), I've finally decided to stop waiting for nVidia drivers and sold my MSI nVidia GTX 1050 Ti 4GB. Fifteen days later, this morning I received my new (used) Sapphire RX 580 Pulse 8GB. In the meantime, I successfully upgraded to Mojave after removing nVidia Web Drivers and unplugging the GTX 1050 Ti, running smooth on Intel® HD4600.

With my current settings, I thought I'd just plug the RX 580 and it'd run straight out of the box... But all I got was some weird blinking/black/no input screens. After reading Replacing GTX 970 with RX 580 and some other topics on Tonymacx86, I tried following their steps.

So I've unplugged the RX 580, booted safely with onboard graphics, downloaded and installed Lilu and WhateverGreen via Clover Configurator (didn't know there was such an easy kext downloader now btw) and rebooted. Well, same weird blinking screen.

Then I stumbled upon this eradicator006's tip on making sure CSM is disabled in the BIOS. Done. When I rebooted, my monitor lost signal for a while and then bumped right into Mojave all of a sudden. It's like the mobo is turning display off during Clover bootloader screen.

I also have dual boot on two SSD's: one for Windows, one for OSX. I can't select Windows 10 due to the screen turning off during loader. Any ideas on what this might be? Did I need to install Lilu + WhateverGreen at all?

Any thoughts would be appreciated. Thank you!

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UPDATE: Just tested onboard HDMI while powering on and it shows up, kinda glitchy. It seems the mobo shifts to RX 580 after Clover boot. Still can't figure it out though.