Place flour in a shallow dish. Whisk eggs and milk in another shallow dish. Mix panko, garlic powder, onion powder, basil, oregano, parsley, and thyme in third shallow dish.
Working with one piece of mozzarella at a time, dredge in flour, shaking off excess. Dip in egg mixture, then coat in panko mixture, pressing to adhere. Repeat steps for a second coating. Transfer to a wire rack set inside a rimmed baking sheet. Repeat with remaining mozzarella.
Transfer rack with mozzarella to freezer and freeze at least 1 hour.
Fit a large pot with deep-fry thermometer and pour in oil to come 2" up sides. Heat over medium-high until thermometer registers 350°.
Working in batches, fry mozzarella, turning occasionally, until golden brown and crisp, 2–2½ minutes. Transfer to a paper towel-lined plate and season with salt while still hot. Serve with Spicy Marinara Sauce.