You know how when Linux boots up, you can press the delete key to jump from the graphical boot screen to a view of the kernel and init system logs? Is this possible with the KDE splash screen so I know what’s going on with that too? I’d be happy with either a splash screen that can optionally be switched to a log view, or just removing the splash screen and having a log view permanently.
The Splash screens seems to be written in QML (check /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/splash/)…so it should be possible to re-write them to display any information you want…including the journalclt output during the login what is probably the closest you get to a log of the KDE startup. Might need you to write a c++ plugin for QML though…no clue if there is already a QML module that can access journald logs.
(And of course you probably want to do that in a local copy in ~/.local/share/plasma/look-and-feel)