Ok I think this is because the lubuntu-qt-desktop package pulls in sddm-theme-breeze which in turn pulls in other kde dependency like plasma-workspace which then cause a conflict with something in lubuntu-qt-desktop. I don't know quite that much debian packaging but my instinct wonders if lubuntu-qt-desktop added a conflicts with sddm-theme-breeze if this error with installs would not happen. Or maybe change the wiki to install a different sddm theme and sddm first.
Ok I think this is because the lubuntu-qt-desktop package pulls in sddm-theme-breeze which in turn pulls in other kde dependency like plasma-workspace which then cause a conflict with something in lubuntu-qt-desktop. I don't know quite that much debian packaging but my instinct wonders if lubuntu-qt-desktop added a conflicts with sddm-theme-breeze if this error with installs would not happen. Or maybe change the wiki to install a different sddm theme and sddm first.