Preheat oven to 425°F (220°C) and position rack in center. Line a large rimmed sheet pan with parchment paper or foil for easier cleanup, or lightly grease with cooking spray.
Pat chicken thighs completely dry with paper towels. In a medium bowl, combine olive oil, garlic powder, smoked paprika, Italian seasoning, onion powder, salt, pepper, and red pepper flakes. Toss chicken in seasoning until evenly coated on all sides.
In a large bowl, toss vegetables with olive oil, garlic powder, Italian seasoning, salt, and pepper until well coated. Spread vegetables in a single, even layer on the prepared pan, leaving space between pieces.
Place seasoned chicken thighs on top of the vegetables, skin-side up, ensuring they are not touching each other to allow air circulation.
Roast for 35-45 minutes, checking vegetables at the 20-minute mark. If zucchini is cooking faster, move it to the outer edges of the pan. Continue roasting until chicken skin is deep golden brown and crisp, and vegetables are tender with lightly charred edges.
Insert a meat thermometer into the thickest part of a thigh (avoiding bone). The chicken is done when it reaches 165°F (74°C) internally.
Remove pan from oven and let chicken rest on the pan for 5 minutes before serving. Sprinkle with fresh parsley and serve with lemon wedges for squeezing over top.