Do not put insulation there.

With the poly already on the wall you would create a warm spot there and could be trapping any moisture that needs to evaporate.

The walls are a little far off the walls but you will be fine with leaving the drywall on. The air space will actually help a bit as well.

Modern insulation on buildings are leaning to exterior insulation and air gaps inside to create another barrier.

I would leave it as is and board