I have two nVidia GTX750Ti cards, each running three 1680x1050 monitors. I can build an xorg.conf that works under Cinnamon and gives me 6 monitors with some minor glitching like I can't drag within Nemo or FF, but overall not too bad. Xrandr doesn't seem to work but I can move things around with the nVidia config tool in any case. This config is using Xinerama, unfortunately, and isn't the fastest thing on the block. Separate Screens seems to give me the ability to maximize a window to just one monitor.

When I boot into Kwin/Plasma5, only the primary display will show anything, though the other 5 monitors show a X mouse cursor (that also does the hopping with loading programs icon!), but they're black. If I bring up a window that homes to the non-primary monitors, I have to right click in the taskbar and Move, then it's pasted to my mouse and I can drag it to the primary window and see it. If I load that config with Xinerama = 0, then just the primary monitor works and gets windows opening on it.

I would go into the many things I've done with xorg.conf to try to figure this out, but I think I need to start from first principles with some people that know what they're doing, because I don't think I'm getting something here. This doesn't seem like it should be as hard as it is and I'm just not getting something about how xorg.conf works and how Kwin interacts with it.

Hoping I can come up with something but even 3 monitors with Plasma is better than 6 with any other DE I've use.

Manjaro Linux is my current distro but this is the behaviour under any distro I've used with Plasma, including Kubuntu, Neon, Mint, etc.