And somehow it fixed itself. I created  /etc/sddm.conf with the following lines:

[Autologin]
User=<my username>
Session=plasma.desktop
Relogin=false

I did shutdown -r now and it continued to come up to a command line prompt, and I'd have to get into KDE by logging in there and doing startx.

Then, instead of simply shutting down, I logged out via the KDE menu, and it took me to the SDDM screen, which I hadn't seen until this point. I initiated a restart from there and when it came back up, it logged me right into my desktop automatically.

I guess something in this process set up a config file somewhere, because it's working fine now.

So, nevermind?