Since sidux days I followed a strict full upgrage procedure.
1. Open a terminal (tty)
2. Drop to runlevel 3
3. apt update
4. apt full-upgrade
5. apt clean (apt autoremove sometimes)
6. reboot
I've lost the drop to runlevel 3 since systemd, but, I start my updates right after a fresh boot, so, I'm assuming I don't need it.
1) Ctrl-alt-f2
2) log in as root
3) apt update (checking what, if anything, is to be removed)
4) apt full-upgrade
5) apt autoremove (Making sure what it's removing makes sense)
6) systemctl reboot
Has worked well with the kde and lxqt versions.