Toast flour in Dutch oven over medium heat, stirring constantly, until just beginning to brown, about 5 minutes.
Whisk in remaining 8 tablespoons butter until melted and combined with flour. Continue to cook, whisking constantly, until deep brown, 4 to 6 minutes.
Add bell pepper, onion, celery, 1 teaspoon salt, and ½ teaspoon pepper and cook, stirring often, until vegetables are softened, 10 to 12 minutes.
Add paprika, cayenne, minced garlic, and minced thyme and cook until fragrant, about 1 minute.
Stir in tomatoes and their juice and cook until dry, about 1 minute.
Slowly whisk in 4 cups shrimp stock until incorporated. Bring to boil, reduce heat to medium-low, and simmer until slightly thickened, about 25 minutes.
Season shrimp with pepper & salt (if shrimp is not pre salted), add to pot, and simmer until cooked through, about 5 minutes.
Stir in scallions and Worcestershire. Season with salt and pepper to taste.
Serve over rice with hot sauce and lemon wedges.
