The overscan setting does not show on this new Mac mini. It did before.

What Display are you using with the new Mac Mini?

The Over/Underscan option may not be available, if you changed the Mac Mini's System Preferences Display Resolution, the cable connection or the Source Ratio of the Display in the Displays Menu settings.

In addition to the Related Article https://support.apple.com/en-us/HT204388 above,

also see > https://support.apple.com/en-us/HT202763

Yea' holding the Alt (option) key and clicking the Scaled button, gives you more display resolutions besides what the system recognizes as defaults for that display.

If it works better for you to select a non-default resolution and then use the Over/Underscan slider, then that is what you need to do.