Combine pasta and liquids in your pot. Add the pasta, broth, and milk to a large pot or Dutch oven. Bring to a gentle boil over medium-high heat, stirring occasionally to prevent sticking.
Reduce heat and simmer until pasta is almost done. Once boiling, reduce heat to medium-low and simmer, stirring frequently, until the pasta is almost al dente and most of the liquid has been absorbed, about 8-10 minutes.
Add the pumpkin and seasonings. Stir in the pumpkin puree, butter, garlic powder, onion powder, nutmeg, and cayenne (if using). Mix until well combined and the butter has melted completely.
Incorporate the cheeses gradually. Remove the pot from heat and add the shredded cheeses a handful at a time, stirring constantly until each addition is melted before adding more. This prevents the sauce from becoming grainy.
Season to perfection. Add the Parmesan cheese, then taste and adjust the seasoning with salt and black pepper as needed.
Finish with herbs and serve immediately. Stir in the fresh sage (or dried if using), saving a little for garnish if desired. Serve hot, while the sauce is at its creamiest.