I just added comment that it was tested on RPi5 8Mb.
@wxl Did you add the link to the Ubuntu instruction? Thx for that.
The lxqt-config-session issue is in fact a little bit odd. Tested it it from the lxqt-config as well in terminal, both failed. Can open lxqt-config-session via sudo but that is of course not the same. Funfact, I could open it from plasma desktop with same user.
Had also a post on this, but no meaningful response…
https://discourse.lubuntu.me/t/cant-open-config-session-settings-from-lxqt-configuration-center/5252/13