qdbus-qt5 org.kde.ksmserver /KSMServer
org.kde.KSMServerInterface.logout 0 2
0
Actually, turns out this now has a more modern alternative (no magic numbers
anymore):
qdbus-qt5 org.kde.Shutdown /Shutdown org.kde.Shutdown.logout
(or .logoutAndReboot or .logoutAndShutdown as necessary)
2T