I very much doubt that the mariadb issue is anything to do with your problem. Every report I’ve seen indicates that this is purely a cosmetic reporting issue and doesn’t lead to a rise in cpu temperature like real cpu load would. I’m running mariadb myself all day every day and my cpu temp right now is 35C which is idle temp.
kactivitymanagerd seems like a good lead. There are reports of it going berserk, seems to be related to it’s (sqlite) database becoming corrupted for unknown reasons. You can try either deleting the whole ~/.local/share/kactivitymanagerd/ folder as suggested above or just ~/.local/share/kactivitymanagerd/resources/ as in this post.