Pound chicken breasts to an even 3/4-inch thickness.
Rub chicken breasts with olive oil and season with garlic powder, paprika, Italian seasoning, salt, and black pepper.
Let seasoned chicken rest at room temperature for 15 minutes.
Cook bacon in a large skillet over medium heat until crispy.
Sprinkle bacon with brown sugar, black pepper, and cayenne in the last 2 minutes of cooking, then drain on paper towels.
Preheat grill pan or outdoor grill to medium-high heat.
Grill chicken breasts for 6-7 minutes per side until internal temperature reaches 165°F.
Let grilled chicken rest for 5 minutes, then slice thickly.
Toast sourdough bread slices until golden and crispy.
Prepare avocado ranch spread by combining mashed avocado, mayonnaise, ranch dressing, lime juice, minced garlic, and salt until creamy.
Spread softened garlic butter on one side of each toasted bread slice.
Generously spread avocado ranch mixture on the buttered side of the bread.
Layer bottom bread slice with lettuce leaves and tomato slices; season tomatoes with salt and pepper.
Add sliced grilled chicken on top of tomatoes, followed by Swiss cheese slices and crispy bacon strips.
Place middle bread slice spread-side down on top, flip, and spread the top side.
Layer with more lettuce, red onion slices, and sharp cheddar cheese.
Add remaining bacon strips and extra chicken if desired.
Top with the final bread slice, spread-side down, pressing gently to hold layers.
Secure sandwich quarters with toothpicks and cut diagonally.
Serve immediately with pickle spears and chips.