Extremely hard layer after cooking the burger on the grill
You did everything right except you overcooked your burgers. Those are relatively thin patties, they won't take 7 minutes a side on high heat, that is what caused that hard crust. I'd be cooking them 3 minutes a side at most. 

A small thing but I'd suggest you replace olive oil with corn, sunflower or another high temperature oil. Olive oil will smoke at a high temperature and add off flavors. This had nothing to do with your thick crust.