The main ublue variants (bluefin/aurora, bazzite) use ublue-update for updating.
This fixes many issues like not updating on metered connections or on low battery.
Have a look in there, maybe you can hook a notify-send -t 0 -a “Update” “Update finished” “reboot any time to apply them”
The -t 0 makes the message stay there forever until clicked. Otherwise you can enter a number in seconds.
You could also open a PR in their ublue upate. They dont want update messages as normies never update manually. So a config could enable this message opt-in.