Add a pacman hook that runs update-notify for each logged-in user with the timer active, and a path unit that users can point at their local repo databases to refresh after aur build.
Each run saves the pending repo, AUR and kernel updates to
$XDG_STATE_HOME/update-notify/pending, independent of the threshold,
so a status bar can show them and drop the ones installed since.
Source ~/.config/update-notify/config to override THRESHOLD, AUR_REPO
and KERNELS, so settings survive package upgrades. The threshold
argument still takes precedence, so drop it from the service. Add a
commented sample config.