Heat olive oil in a large saucepan or Dutch oven over medium-high heat. Cook and stir sausage and garlic in hot oil until browned and crumbly, 5 to 7 minutes. Transfer sausage and garlic to a plate using a slotted spoon, leaving any drippings in the saucepan.
Saute onion, green pepper, sweet pepper, and salt in the same saucepan until slightly softened, about 5 minutes. Add zucchini and continue cooking until onion is translucent, 5 to 10 minutes more. Drain off any excess liquid.
Stir sausage, crushed tomatoes, tomato sauce, diced tomatoes, tomato paste, oregano, basil, rosemary, parsley, black pepper, and red pepper flakes into onion-zucchini mixture. Bring to a simmer, reduce heat to low, cover the saucepan with a lid, and cook, stirring occasionally, until flavors blend and sauce is thickened, about 2 hours.