Regarding notifications for updates I will ask again in Support channel soon as a follow-up to the question you had asked there. Tracey had said that in place for the deprecated solus-update-checker, discover/GNOME-Software would notify for updates. Can't say regarding discover, but for GNOME Software this seems only to be true in GNOME itself, not in niri. In GNOME, GNOME Software seems to be started in the background, at least I can kill it without having it opened, which is not the case for niri. Haven't figured out yet if it can be started in background in niri and how this whole packagekit thing works. And actually, I don't want it to be started in background as I dislike it.
Also I don't know whether Tracey's statement means that people who don't wanna use neither discover nor GNOME-Software and prefer cli can't have update notifications.
I figured out so far that pkcon refresh force refreshes the packagekit cache or whatever and that then pkcon get-updates --plain shows whether updates are available. So theoretically possible to build a script with those commands that then triggers notification.
But I do not know if it's save to run pkcon refresh force and if this is usually run by discover/GNOME Software themselves or some packagekit service and how often this is refreshed usually. I didn't find a setting for check for updates intervall in GNOME Software. Also, GNOME Software notification behaviour is weird.
Pretty sure Joey or sheepman are familiar with that whole packegkit stuff and can give advice what would be the best course of action here for notifications.
Edit: you are on Budgie and hence probably using discover? Is it showing notifications for you for updates without having it run manually prior?
Edit: I mean when in niri