I prefer using Command Output with a simple shell script including several functions instead of plasma-discover notifier.
It displays a icon that indicates the system status (whether are pending software updates or not), and instantly lists pending updates upon mouse hover.
And when clicked, it offers options to take a system snapshot with Timeshift, and to perform the updates online or offline.
However, this is a very basic solution with many shortcomings, although I have obvious reasons to prefer it over the Discover Notifier.
But, it seems someone with proper software skills have finally developed a much better alternative to Plasma Discover Notifier in the form of a plasmoid called Kempt.
It currently includes only one backend, which is dnf, but the developer describes it as “Tidy system updates for the Plasma desktop: a tray widget and a CLI over one engine, for dnf and Flatpak. Built to grow into a universal Linux updater.”
Probably some limitations of pkgcli made it impossible to use instead of interacting directly with the package management backend, and therefore, it needs contributions to make this promising plasmoid useful to all KDE Plasma users regardless of their distro preference.
On the other hand, development of pkgcli and plasma-discover notifier in this direction could perhaps be the ideal solution in the long-term.