I really want to get highlights. My hair is a medium brown and it looks a lot better when its a light brown to a dirty blonde(my hair used to be this color and it got darker over the years). I got full head highlights two years using the foil method, but I'm not sure if it ruined my curls because I was wearing my hair straight. I want to get highlights again in september using the foil method. I won't straighten my hair again I'll keep air dry it.

How damage is my hair going to get from getting highlights? And if I keep touching up the roots will I ruin my curls?
3a CG
Cleanser: shea moisture's african black soap cleanser or moisture retention shampoo
Conditioner: tresemme naturals, shea moisture restorative conditioner
Syler: shea moisture's curl soufflé and smoothie.