EOS is arch-derived, yes. It's basically just arch-but-preconfigured to be entirely honest. It gets all its stuff directly from the arch repo (unlike, say, Manjaro which has its own separate repo) and then just adds its own repo with some extra packages.
And speaking of -- The Arch repo hasn't updated to Plasma 6 yet (I am on the hypetrain for Plasma 6, I would have known!), or at least, hadn't by last night.
As for journalctl -- I did check, and there is something error-shaped relating to sddm -- Here's a pastebin that contains the error and stuff.
I did realise -- Like, right now -- I had SDDM-git installed rather than regular SDDM on my machine (don't even remember why I did that. I did say on OP that I was incompetent at package management) -- So I just changed that and will attempt a reboot cycle later today (am working rn). Will post results.
BWAHAHAHAHAHAHAHAH
In the MLP fandom "Twaggle" or "Twiggie" refers to an intentionally badly drawn version of Generation 4 Series Protagonist Twilight Sparkle. And she is often depicted just generally being a little shit of an annoying child
here are some some examples