Cut each mozzarella string-cheese stick crosswise in half to make 16 pieces.
Whisk the eggs with 1 Tbsp water in a shallow bowl.
Place the flour in a second shallow bowl and season it with salt and black pepper.
Combine the seasoned fine breadcrumbs, panko, vegetable oil, and garlic powder in a third shallow bowl.
Dip each cheese piece into the egg, coat it in flour, return it to the egg, and press it into the breadcrumb mixture. Cover the sides and both cut ends completely.
Arrange the breaded pieces in one layer on a freezer-safe baking sheet or flat tray. Freeze for at least 30 minutes; allow approximately 40 minutes in the total recipe schedule.
Preheat the air fryer to 390°F/approximately 200°C.
Generously coat the frozen mozzarella sticks with cooking spray. Arrange them in the basket in one uncrowded layer, working in batches when necessary.
Air-fry for 5–7 minutes, checking at approximately 4 minutes. Remove when the coating is crisp and golden and the first small bead of mozzarella begins to appear.
Serve immediately, with marinara if desired.