Bring a large pot of salted water to a boil and cook the spaghetti according to package directions until al dente; drain and set aside.
Meanwhile, heat a large pot or Dutch oven over medium heat and add the ground turkey, Italian sausage, and chopped onion.
Cook, breaking up the meat with a spoon, until the meat is browned and the onion is softened, about 6–8 minutes; drain any excess fat or liquid.
Add the tomato sauce, fire-roasted diced tomatoes, Italian-style diced tomatoes, tomato paste, dried Italian seasoning, garlic powder, salt, dried oregano, dried thyme, and dried basil to the pot; stir to combine and bring to a simmer.
Reduce heat to low and simmer the sauce, uncovered, for 20 minutes, stirring occasionally.
Stir in the tablespoon of white sugar, taste and adjust seasoning if desired, then toss the sauce with the drained spaghetti or spoon the sauce over plated pasta and serve immediately.