Yes, it is a shame the jump between sizes is so big. I agree the best fit for you would probably be between 20 and 22.5.

The height changes more than the length when you go up a size so generally a larger size will put you in a more upright position. Which Trek are you looking at? If you are comfortable on a 20" Schwinn, you might prefer the same size Trek. If you want to sit a little more upright the 22.5" might suit you better.

I'm 6'1" and ride 20.5" hybrid, 19" MTB. I think if I was your size the 22.5" would be better for me, but we're all different. Try them on and see which fits. Changing a seatpost or stem to fine tune the fit is not unusual.