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It would be a nice time-saving feature to let the user specify the time to shutdown and to link the activation of the process with a user-configurable keyboard command.

Let the user configure the number of seconds to shutdown, reboot, and suspend.
Let the user specify a custom keyboard shortcut to activate them.

The advantage is that it allows a user on the move to shut the system down very quickly without having to create a custom shutdown shortcut.

A custom keyboard shortcut is something I used for 20 years when I was using Windows. Then I started doing it the same way in Kubuntu before learning that the way I was doing it was potentially harmful to my system because it simply killed CPU processes without going through the proper OS log out and shut down processes. My problem was partially solved on the Kubuntu forums, but I still can't safely execute a rapid reboot or suspend with a keyboard shortcut.

For details of that discussion see this thread:
https://www.kubuntuforums.net/showthread.php/75913-Shutdown-delay-x-number-of-seconds

You should be able to do all of this using the existing functionality:
To set up shortcuts to the direct functions (as in: shut down/reboot now, instead of with delay), use System Settings -> Shortcuts -> Global Shortcuts -> Power Management.
To set up shortcuts to custom commands (as in: shut down/reboot with some delay), use the Custom Shortcuts page to create shortcuts to your custom commands.

From the global shortcuts menu, it is possible to open up the Shut Down screen with the default 30 second count down. Which isn't what I was suggesting.

The other options in the global shortcuts that I tested were KShutdown and ksmserver, which didn't do anything that I could see.

And while it is possible to set up a shortcut to execute the shutdown command, doing so just kills everything; which unless I'm mistaken, is potentially harmful to the desktop environment. (see post #3 in the OP link) - And I could not find a documented way to set it to execute in seconds. The only documented time options that I found were instantly, and increments in minutes.

What I am suggesting would be an easy way in the GUI to change the 30 second default timer to some other value in seconds and a way to call up that screen with a different pending action (Reboot, Shutdown, etc) via user configurable key strokes.

(There might be a way to do that with existing functionality, but it isn't intuitive or easy to find.)

What is the intent of changing the default timer, or otherwise shutdown with some delay?

Reviewing the thread in your OP, I see that you were shown the line to use qdbus to invoke the safe shutdown, so even if the global shortcuts for shutting down don't work properly (which that may be a bug somewhere that should be fixed), you can set up a Custom Shortcut to invoke the d-bus call to shut down. So, really the delay is the only component that's missing, and I would agree that would be nice to be able to customize (in general, not necessary tied to any particular shortcut).