First off I would say check the normal ‘plasma is wonky’ things:
https://wiki.archlinux.org/index.php/KDE#Configuration_related
(possibly in conjunction with the above tips like making sure not to use git packages … and afterwards clearing out the cache and such)
If you absolutely must create a new user … then I made a tool for such migrations (well really for in between installs/systems … but it works here)