Today it wanted me to reboot (again!) for a new update (I had already rebooted it yesterday for 2025030800).
Wasn't sure if it was a glitch or not until I came here just now and saw there was a 2025030900, and I just checked the phone and that's what's now installed.
In other words, suddenly it works properly now.
Just to check into everything I tried to check into GrapheneOS's suggestion.
Go into Settings > Apps, turn on showing system apps, find the System Updater app with search and turn notifications back on. There's also a shortcut to that menu from Settings > System > System update. Perhaps you meant to turn off the "Already Updated" notification channel but you turned off all notifications from it instead.
I don't, in Settings > Apps, see a way to turn on "Showing system apps" and system update app does not show up in the "all apps" list, so I wasn't sure what to do here.
I'm going to figure the same thing happened to me as with goslife--he reports exactly the same symptoms.
Thanks to everyone for their assistance...other than the occasional difficulty finding out where a setting is, and some quirks I'll post about one by one, I'm liking this system (and it's great telling Google what to go do with itself).