I did some experiments with the set fade out and blinking (0x23) command to see if it might be useful to add a blink screen function to the Arduboy2 library. It turned out to have the same limited brightness range as the contrast command. (It likely uses the same hardware.) So, I didn’t bother doing it.
For the same reason, as you discovered, I didn’t feel it was useful to add a contrast control function to the library.
@Dreamer3 (aka yyyc514 on GitHub) did some work a while back on using a combination of contrast and charge pump control to extend the range of brightness control. I haven’t looked at how successful he was or how well it works.