As you have already learned, there is an AUTO feature in Appearance which determines when Dark and Light Modes are set. This is based on your location and then Sunrise and Sunset times come into play. This is all done automatically. You would like some manual control on the timings of the switch from Dark Mode to Light Mode and vice versa.
There is no straightforward way in Preferences to set a schedule (like Night Shift) as you have discovered. I have run into a workaround for this and is located here ➔ How to enable Dark Mode on Mac automatically on a schedule . The workaround takes a bit to set up correctly and am not sure if it is worth the time for you. Review it and see if it works for you.
Axel F.