Thank you,
@CaseySJ, and many other contributors, for an invaluable guide!!! With your help I have a great system. I started with a RX 590, then tried a 5700 XT, but eventually settled on a 2080 Ti. I thought briefly about running 2 cards but decided against it because of thermal concerns. At the moment, I am using the 9900k's iGPU for my hack, via the Designare's HDMI port. I briefly ran into major trouble before I realized that the HDMI v1.4 port is limited to 4k30. I was caught in framebuffer purgatory and was only able to recover by reinstalling one of the AMD cards. At this point, those cards are no longer with me, so I am being very careful to avoid potentially problematic settings.
I am currently running at my monitor's native 1440p resolution, non-retina. I have both Thunderbolt ports tied up so I am unable to use them for video. I have been trying to figure out if there is anyway of adding video ports for the iGPU to utilize. As far as I understand, my bottleneck is the v1.4 HDMI port on the Designare. The iGPU AFAIK is capable of spitting out a good deal more than 4k30. Is there any way to add a card or adapter that would allow an HDMI v2 or DisplayPort output? Ideally I'd like to be able to utilize a retina resolution as close to (looks like) 1440p as possible.
Thank you!