Cook the chopped bacon in a large Dutch oven over medium heat until crisp, 8 to 10 minutes. Transfer to a paper towel-lined plate with a slotted spoon and set aside. Leave about 2 tablespoons of the bacon fat in the pot (add the butter if you do not have enough fat).
Add the onion, carrots, and celery to the pot and cook in the bacon fat for 5 to 6 minutes, stirring occasionally, until softened. Add the garlic and cook for 1 minute more, until fragrant.
Sprinkle the flour over the vegetables and stir for 1 minute to coat. Slowly pour in the chicken broth while whisking constantly, and keep whisking until the mixture is smooth with no lumps.
Stir in the shredded chicken and the ranch seasoning. Bring to a gentle simmer and cook for about 10 minutes, until the soup has thickened slightly.
Add the egg noodles and cook, stirring occasionally, until just tender (al dente) according to the package directions. Do not overcook them — they will continue to soften in the hot soup.
Reduce the heat to the lowest setting. Add the softened cream cheese a few cubes at a time, stirring until each addition has melted before adding more. Stir in the heavy cream.
Add the shredded cheddar a handful at a time, stirring until melted and smooth after each addition. Stir most of the crispy bacon back into the soup, saving some for garnish. Taste and adjust the salt and pepper. Keep the heat low — do not let the soup boil once the dairy is in.
Ladle into bowls and serve hot, garnished with the reserved bacon, fresh parsley, extra shredded cheddar, cracked pepper, and sliced green onions if using.