should charge balance be based on activities?
No, charge is not really a activity based property.  It is a number.  Just as the activity coefficients don't change the number (or moles) of potassium atoms or phosphate anions, they don't change the charge either.

If you told me that one liter of a 1.0 molar potassium chloride solution had activity coefficients of 21 for potassium and 186 for chloride, then the amount of charge in the solution is still one mole of potassium cations and one mole chloride anions.