I’m really not loving the idea of using a second action just to notify the user of the need for restart, and I don’t want to broadcast what’s being patched.
Don't use the Post Action Tab, use the Messages Tab.
In the action, use
restart 1500
Which is 25 minutes.
There are some things to consider... Do you want the message to display during the countdown (of 1500 seconds) or do you want to display a message before the reboot occurs. You can set the restart to command with no seconds listed, or however many seconds you want and then set the message tab to set a 15 or 30 minute deadline or you can set the deadline to one minutes and set the restart command to 1440 seconds.
If it were me, I would set the restart command to 5 minutes (300 seconds) and then set the message to have a deadline of 15 minutes, and this is because if the timer to reboot (restart to command) is under 5 minutes, it actually counts down by seconds.
In the case below, I set the deadline to 1 minute but I also set the restart command to 500 seconds, so it will show a 60 second countdown and then kick off the action which will set off a 500 second countdown.
if you choose to do it this way and you allow canceling of the action by the end user, they will only be able to cancel during the 60 deadline warning. They will not be able to cancel after the action (restart 500) is executed.