Mix olive oil, paprika, oregano, salt, and pepper for marinade in a glass bowl.
Begin paella: Add chicken and stir to coat. Cover and refrigerate until needed.
Heat 2 tablespoons olive oil in a large skillet or paella pan over medium heat. Stir in garlic and pepper flakes, then stir in rice. Cook and stir until rice is coated with oil, about 3 minutes.
Add saffron threads, bay leaf, parsley, chicken stock, and lemon zest. Stir until well combined and bring to a boil. Reduce heat to medium-low, cover, and simmer for 20 minutes.
While the rice is cooking, heat olive oil in a separate skillet over medium heat. Stir in marinated chicken and cook for 3 minutes. Add onion and cook until translucent, about 5 minutes.
Add bell pepper and sausage; cook and stir, breaking sausage up with a spatula, for 5 minutes. Add shrimp; cook and stir until shrimp are bright pink on the outside and the meat is opaque, about 2 minutes.
Spread rice mixture onto a serving tray. Top with meat and seafood mixture.