Dice the onion and celery into 1/4-inch pieces, peel and dice the carrots to match, mince the garlic, and dice the bacon into 1/2-inch pieces. Shred the chicken, grate the cheddar, and cube the softened cream cheese; measure the heavy cream and let it sit at room temperature.
Cook the diced bacon in a large Dutch oven over medium-high heat for 6 to 8 minutes until crisp and golden, then transfer to a paper-towel-lined plate with a slotted spoon. Leave about 1 tablespoon of bacon grease in the pot and reserve a third of the bacon for topping.
Reduce the heat to medium, add the butter to the bacon grease, and stir in the onion, celery, and carrots. Cook for 5 to 7 minutes until soft and translucent, scraping up browned bits from the bottom of the pot.
Add the minced garlic and cook for about 30 seconds until fragrant, then sprinkle in the ranch seasoning and black pepper and stir for 20 to 30 seconds to toast the seasoning and coat the vegetables. Do not add salt yet.
Pour in the chicken broth and bring to a gentle simmer over medium-high heat, then add the shredded chicken and the larger portion of cooked bacon. Simmer for about 5 minutes until the chicken is hot and the flavors have blended.
Turn the heat down to low and stir in the cubed cream cheese for 2 to 3 minutes until it softens into the broth. Add the shredded cheddar a handful at a time, stirring after each addition until the soup is smooth, creamy, and slightly thickened.
Add the baby spinach in two or three handfuls, stirring as it wilts, and cook for 2 to 3 minutes until soft and bright green. Slowly pour in the heavy cream while stirring, then heat for another minute or two without letting the soup boil.
Taste and add salt only if needed, then ladle into warm bowls. Top each with the reserved bacon, extra shredded cheddar, sliced green onions, and fresh parsley if you like, and serve right away.