Place salt, lemon zest, and 3 cloves garlic in bowl of a mortar and pestle. Pound with the pestle until mixture begins to form a paste, about 2 minutes. Add chopped basil, parsley, oregano, and thyme and pound with pestle until mixture begin to come together, about 5 minutes.

Drizzle about 1 tablespoon of the olive oil into herb mixture. Grind together until mixture begins to form a sauce for marinating, about 1 minute. Pour in the remaining 3 tablespoons olive oil. Stir mixture with a spoon until mixture is thoroughly combined, adding additional olive oil as needed. Mixture should be fairly thick but pourable.

Place shrimp in a large bowl and mix in about 2/3 of the sauce, reserving 1/3 for serving. Stir until shrimp are evenly coated with the sauce, about 2 minutes. Transfer shrimp to a resealable plastic bag. Refrigerate 2 to 3 hours. Cover and refrigerate remaining sauce.

Preheat an outdoor grill for high heat and lightly oil the grate.

Thread shrimp onto skewers (pierce each twice, once through large part of shrimp, once through small part). Place skewers on hot grill. Cook on each side until shrimp are bright pink and opaque and exterior is beginning to caramelize, 2 to 3 minutes per side. Transfer skewers to serving platter.

Pour remaining sauce into mixing bowl. Whisk in 1 tablespoon olive oil, lemon juice, red pepper flakes, cayenne pepper, salt and black pepper. Spoon sauce over shrimp. Serve with lemon wedges.