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.
While the pasta cooks, heat a large pot or Dutch oven over medium heat. Add the ground turkey, Italian sausage, and chopped onion.
Cook, breaking up the meat, until the turkey and sausage are 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, oregano, thyme, and basil to the pot; stir to combine.
Bring the sauce to a boil, then reduce heat to low and simmer, uncovered, for 20 minutes to meld the flavors.
Stir in the 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.