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.