Add shredded chicken to the slow cooker.
Add dry rigatoni pasta and distribute evenly on top of the chicken.
Add 2 cups chicken broth, minced garlic, oregano, thyme, Italian seasoning, red pepper flakes, and 2 jars of Alfredo sauce. Stir well to combine.
Cover and cook on high for 30 minutes. Stir in drained sun-dried tomatoes. At this point, check on the sauce, if it's too thin for your liking, stir together in a small cup, the cornstarch, and water with a fork until smooth. Add the mixture to the slow cooker and stir.
Cook for another 15 minutes on high. Check on pasta, if it's still uncooked, cook it for another 15 minutes. Usually, it takes 45-60 minutes for the pasta to be done, but it depends on the slow cooker, so keep an eye on it.
Place slow cooker on Keep Warm (or on Low if you don't have that setting), taste and adjust for salt, pepper, and spiciness. Stir in fresh spinach and cover for 10 minutes, the heat will wilt the spinach.
Gently stir to combine and serve immediately.