Thanks for the response Iowagriz.
I emailed Salsa, and they don't seem to think it's possible to get a 17 inch low gear. It makes no sense to me, but apparently the 1x setup will provide the lowest gear with a 42T max cog (which is still over 20 gear inches). I don't know if that has to do with the fact that the 2020 Cutthroat employs boost spacing (yet the max tire width is the same?).