Also install qt5 sddm components
When a qt5 theme is specified in sddm config, the qt5 greeter will be used, which is not functional due to the lack of these components, breaking sddm completely.
Installing the qt5 sddm components allows qt5 sddm themes to work. However, it's worth noting that setups with a qt5 sddm theme and GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=layer-shell
are still broken, due to the lack of layer-shell-qt5 in arch repos.