Well let's see...have you mastered the Nexstar? can you fly a roll without losing altitude? how about 2 in a row?....can you fly the pattern inverted? do you grease in 90 % of your landings? if you answered no to any of the above then more time on the Nexstar.
I would not recommend expo is this can become a crutch, expo is more for 3D type aircraft with large control surfaces.....to lessen the sensitivity on your controls set up dual or triple rates......to start with dual rates make the instruction recommended low rate throws your high rate and your low rates reduce by a 1/3.....
Good Luck