Season the chicken: Pat chicken dry.
Sprinkle with Italian seasoning, garlic, onion powder, salt, black pepper, and crushed red pepper if using. Place in the slow cooker.
Add liquids: Pour in the chicken broth. Dot the chicken with cream cheese cubes.
Cover.
Cook low and slow: Cook on Low for 3.5–4.5 hours or on High for 2–3 hours, until the chicken is easily shreddable and the cream cheese is softened.
Shred the chicken: Remove chicken to a plate and shred with two forks. Return it to the slow cooker and stir to combine with the melted cream cheese and broth.
Finish the sauce: Stir in heavy cream (or half-and-half), Parmesan, and olive oil or butter. Taste and adjust salt and pepper.
If using add-ins like sun-dried tomatoes or spinach, stir them in now. Cover and keep on Warm.
Cook the pasta: Boil pasta in salted water according to package directions until al dente. Reserve 1/2 cup pasta water, then drain.
Combine: Add drained pasta to the slow cooker and toss until well coated.
If the sauce seems too thick, splash in reserved pasta water a little at a time to loosen.
Serve: Top with extra Parmesan and chopped parsley or basil. Serve hot with a side salad or garlic bread.