As far as I'm aware there's no way to change this value because a Mac must be awake for at least 10 minutes past the scheduled time. It's because of this that the 10 minute value is set and cannot be altered, although I suppose there's no reason why it couldn't be extended beyond that (such as the 30 mins you want). Regardless, I'm not aware that this can be done.
According to Apple:
To shut down automatically, your Mac must be awake at the time that
it's scheduled to shut down, and remain awake for at least 10 minutes
past that time. If it's sleeping at the scheduled shutdown time, it
continues sleeping instead of shutting down. If it's set to go to
sleep after less than 15 minutes of inactivity, it might go back to
sleep before it can shut down. To make sure that your Mac shuts down
even when sleeping, set it to start up or wake 5 minutes before the
scheduled shutdown time.
Source: Set a startup, wake, sleep, restart, or shutdown time on your Mac
One thing you could do is check the man page for
pmset on your system to double-check, as different versions of the OS will provide different options (and sometimes particular hardware can offer different options depending on it's type and location).
To check the man page for
pmset on your system, launch the terminal and enter
man pmset. Note this may take a while to retrieve the info.