My desktop is configured in such a way that a sudden power loss (or abrupt restart) would result in absolutely no data lost. I therefore want to make sure that updates can always install on schedule and that I don't have to manually close any programs when clicking the Restart button.
How do I force Mac to always restart without delay, completely ignoring whatever software might be running in the background?
In my experience the best way to keep apps from blocking an update install is to just close all GUI apps before going to bed - or putting my machine "to sleep".
I've also noticed that I need to logout of Terminal, especially.
There's really no "non-hacky" way of doing way you want.
You have a really special use case which is extremely rare. If you really want to do this, you should hook up a script or similar to detect when the automatic update window comes up and let that trigger simply closing all your programs (for example with the "kill -9" command).
If you just want to force a reboot, you can do that with the "shutdown -h now" command from the command line, but that will not ensure that updates are automatically installed.
You can refer to this answer to see how you can create an easy way of force rebooting the system no matter which GUI programs, you have running:
How to prevent apps from from not letting you to restart Mac OS
