Preheat the oven to 350°F. Cook the spaghetti to al dente, drain, and set aside.
Cook the bacon if needed, then cool and chop.
In a very large mixing bowl, combine the spaghetti, cream of chicken soup, cubed Velveeta, warmed chicken broth, milk, ranch mix, and cooked chicken. Stir until the sauce starts to come together.
Pour the mixture into a 9x13 baking dish and spread evenly.
Top with shredded cheddar, then sprinkle chopped bacon over the cheese.
Bake uncovered for 30 minutes, until bubbling around the edges and melted on top.
Let rest for 5 to 10 minutes before serving.
Notes
Cook the pasta 1 to 2 minutes shy of package directions so it does not get mushy in the oven. Warm the chicken broth so the Velveeta melts more easily. Cook the bacon before adding it to the casserole. Let the casserole rest before serving so the sauce thickens.