Pat fish, shrimp, and scallops dry with paper towels and refrigerate until needed. Dice onion and celery into uniform ¼-inch pieces, mince garlic, cube potatoes into ½-inch pieces, and chop bacon into ½-inch pieces.
Cook chopped bacon in a Dutch oven over medium heat for 5–7 minutes, stirring occasionally, until crispy. Remove with a slotted spoon to a paper towel-lined plate, leaving about 2 tablespoons of fat in the pot.
Add butter to the pot and melt, then add onion and celery; sauté 5–6 minutes until softened and translucent. Add garlic and cook 1 minute more, seasoning with a pinch of salt and pepper.
Sprinkle flour over the vegetable mixture and stir constantly to coat. Cook for 2–3 minutes until the raw flour smell is gone and the mixture looks dry and clumpy.
Gradually pour in seafood stock about 1 cup at a time, whisking after each addition, then stir in the heavy cream, bay leaves, thyme, smoked paprika, and cayenne. Bring to a gentle simmer over medium-high heat.
Add cubed potatoes, reduce heat to medium-low, and simmer uncovered for 15–18 minutes, stirring occasionally, until potatoes are fork-tender. Taste and adjust seasoning.
Reduce heat to low, gently nestle fish pieces into the chowder and cook 2 minutes, then add shrimp and scallops; cover and cook 4–5 minutes until shrimp are pink and opaque and scallops are just firm. Remove and discard bay leaves.
Adjust seasoning and add a squeeze of lemon juice if desired, then ladle into bowls and top with reserved crispy bacon, fresh parsley, and cracked black pepper. Serve with oyster crackers or crusty bread.