I'm also having intermittent problems with videos looking scrambled, even locally. How can I test my gpu? Clover folder attached. I also ran Cinebench and got 3537 points...
However, my system is not stable. Every couple hours I'll be doing something innocuous and everything will freeze up in the worst way. I can't even force quit, it's systemic.
This was my working Efi with the RX580. I used it with iMac18,3 and iMac19,1 (I use no onboard Audio and a different Bluetooth chip). I changed the Serial Numbers.
The differences I found are:
1) I disabled HDAS to HDEF
2) I have less entries in Boot Arguments. I used no shiki with the 580 and im not sure if you really need agdpmod. I also never needed keepsyms=1
3) different platform-id in Devices/Properties (headless, I hope your iGPU is set to ON (BIOS) and NOT connected!!)
I would try with my config to look if it works and then add your additional entrys one after the other, to see where the problem came from.
The rest of your Clover folder (I hope it is called "EFI" on your Partition) seems OK to me.
Do you have drivers installed in Library/Extensions as well? Personally I would not install anything there. But if, make sure that they are the same as in your EFI/drivers/UEFI folder and also not more than there.
By the way, I never used the HDMI and DVI Outs. Only Displayport.