My guess is that I just installed things in the wrong order and it generated a bad config somewhere (e.g. installing plasma-desktop before x or creating a user account without the correct permissions. I don't really know.)
I started with:
sudo pacman -Syu plasma-desktop plasma-wayland-session kde-applications
Edit: I actually just ran the command and can now load plasma on X11. Weird. I installed some other X packages after xfce started working, so maybe that had something to do with it.
Running plasmashell complained:
Could not load the Qt platform plugin “xcb” in “” even though it was found.
Thanks for the tip! It made me aware of missing locales, which is probably why I was having issues with i3 as well.