Thank you for your help and suggestions! Disabling session restore resolved the shutdown issue.
However, I’m still encountering the partition access problem, which I believe may be specific to KDE Plasma. When I open Dolphin, the partitions appear in the sidebar. But when I try to mount a partition by clicking on it, a popup appears requesting authorization, so I need to enter my password the first time. This has never happened in Linux Mint.
If anyone has insights on how to adjust permissions to allow access without requiring authentication, I’d appreciate it.
What I do is make mountpoints, take ownership of them, and add them to /etc/fstab if regular access is desired. For external devices I’ve not had any issues mounting on-the-fly with e.g. Dolphin or, more usually, when the KDE notifier alerts me devices are available, and I mount from there?
… Sorry, just my tuppence worth, but not had to do anything with polkit rules on my systems., something must have been changed?