Oh, forgot to mention: The RX 5600 XT was returned. Too much display corruption. I'm guessing that, while PCIe should in theory be forward-compatible, it really isn't (my motherboard has 3.0, the card was 4.0).
Instead, I got an RX 580. Now this is working much better. Still a few quirks, though I'm chalking those up to KScreen in KDE. Seems to forget a display exists if it's DisplayPort and the monitor falls asleep, for instance. But, mostly has been working admirably.
The very tiny home of CaptainSpam and all the various goings on at Exclaim Industries (which is just CaptainSpam).