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