Add the chicken to the slow cooker.
Pour in the marinara and chicken broth. Stir in garlic, onion powder, Italian seasoning, red pepper flakes (if using), salt, and pepper.
Scatter the cream cheese cubes on top. Cover and cook on Low for 4–5 hours or High for 2–3 hours, until the chicken is cooked through and tender.
About 20 minutes before serving, cook the pasta in salted boiling water until just shy of al dente.
Drain and set aside.
Shred or chop the chicken right in the slow cooker using two forks. Stir until the cream cheese is fully melted into the sauce.
Stir in the Parmesan cheese and butter (if using). Add 1/2 cup milk or half-and-half to loosen and enrich the sauce.
Adjust with more milk as needed to reach your preferred consistency.
Fold in the cooked pasta. Toss until the pasta is well coated and the sauce is silky. Taste and adjust salt and pepper.
Serve hot, topped with extra Parmesan and chopped basil or parsley.