Stir breadcrumbs and milk in a bowl. Let them soak for 5 minutes.
Combine turkey, soaked crumbs, Parmesan, parsley, garlic powder, onion powder, 0.5 tsp salt, and 0.25 tsp pepper.
Form 1-inch meatballs with damp hands. Set on a plate.
Warm 1 tbsp oil in a large pot over medium heat. Brown meatballs in batches, 4–6 minutes total. Transfer to a plate.
Add remaining oil. Sauté onion, carrots, and celery with a pinch of salt until softened, about 5 minutes.
Stir in garlic and Italian seasoning. Cook until fragrant, 30 seconds.
Pour in broth and scrape the pot. Return meatballs and any juices. Simmer gently for 10 minutes.
Stir in orzo. Simmer, stirring occasionally, until al dente, 8–10 minutes.
Fold in spinach and lemon juice. Season with remaining salt, pepper, and red pepper flakes.
Rest 2 minutes to thicken slightly. Ladle into bowls and finish with parsley and Parmesan.