For some reason, whenever booting from Grub, I’m presented with a console only login screen. I can log into it, but I don’t know how to actually get out of it and into the desktop environment (KDE).
To start Plasma with xinit/startx, append export DESKTOP_SESSION=plasma and exec startplasma-x11 to your .xinitrc file or run directly in the console startx /usr/bin/startplasma-x11. If you want to start Xorg at login, please see Start X at login.
SDDM is the default display manager for handling that. For some reason on your setup, it’s not starting. You might review the SDDM Wiki page and compare the recommended configuration to your own, to see what might be amiss.
Alright, initally I was just using startx /usr/bin/startplasma-x11 as @Bink suggested, but eventually after months of it, I was starting to really miss it actually going automatically straight to the GUI as it used to before. So this was the command that helped with that:
systemctl set-default graphical.target
Since before I used the command, it turns out the default target to boot into was multi-user.target, which apparently wasn’t the right one.
I guess it just took a handful of months to actually force myself to fix it, but yeah it’s now fixed so that’s good.