Your journal output might be too curated, I don't see anything that would keep the display manager from starting up.
Then after following some suggestions from users in r/Kubuntu I’ve made a bit of progress.
We'd need more info on this, like what exactly did you change and what was that progress and how/when did it turn into regress.
Journal output for SDDM and related services, and for the relevant timeframe, would be better.
I suppose SDDM is the default for Kubuntu, and if you had done other possibly relevant things to your setup you'd have told us.
FWIW, it could be GPU related, but that really just is a wild guess.
PS: identical post here: https://lemmy.sdf.org/post/43982607