The others are right.
Making the wing thicker just adds drag.
The full size makers use thicker wings because it is easier to make them stronger, but any more than 12% thick is just extra drag to reduce the top speed.

If you want a slower stalling speed you can either add camber to the wing (use a Clark Y section or Selig 3021) or, if you want to keep the inverted flight capabilities, temporary camber in the form of flaps. I doubt if the tail is big enough to cope with the pitching effect of flaps.

The only way left is to keep the wing loading low. The lower the wing loading the slower the stalling speed.
Alasdair