gk7ncklxlts99w1 Only the notifications that appear on a profile's lock screen are sent to other users. So, if you set the Already up to date and Error reporting to Silent and block them from appearing on the lock screen, they won't be sent as notifications to other users.
You can also leave the Reboot prompt as-is; that way when you have silenced the other updater notifications and get a notification from System updater from the Owner profile, you know it's a reboot prompt for an update.
You can silence the update and error channel notifications in Settings > System > System updates > Notification settings > [Error reporting / Already up to date]. In there, set the notifications to Silent and Lock screen to Don't show notifications at all. That should silence the spam.
Hope this helps & good luck.