Whisk together olive oil, soy sauce, balsamic vinegar, lemon juice, brown sugar, and Dijon mustard in a bowl. Add dried oregano, basil, thyme, paprika, garlic powder, onion powder, salt, pepper, and red pepper flakes.

Reserve 3 tablespoons of marinade, and coat chopped zucchini, bell peppers, and red onion in the remaining marinade. Refrigerate for 30 minutes.

Cut chicken into equal-sized cubes, combine with remaining marinade in a zip-top bag or bowl, and refrigerate for at least 4 hours.

Soak skewers in water if wooden. Thread marinated chicken and veggies onto skewers, alternating pieces.

Let skewers rest at room temperature for 20-30 minutes. Preheat grill to medium-high or oven to 400°F. Cook skewers for 8-12 minutes on grill or 16-22 minutes in oven.

Once cooked, let kabobs rest for 5 minutes and optionally drizzle reserved marinade before serving.