Place all dry rub ingredients into a small bowl and whisk together until completely combined.
Rub brisket all over with dry rub. Refrigerate for 30 minutes.
Pour 2 cups ketchup, 3/4 cup brown sugar, vinegar, 1 tablespoon Worcestershire sauce, garlic powder, onion powder, cumin, ground mustard, and black pepper into a slow cooker and stir together.
Place brisket into slow cooker, turning to coat. Cover slow cooker and cook on high for 5 hours.
Remove brisket onto a foil lined baking sheet. Loosely cover with foil and set aside.
Skim as much fat as possible from sauce and pour reserved sauce into a small saucepan, discarding fat.
Stir in remaining ketchup, sugar and Worcestershire sauce and simmer for 15 minutes until sauce thickens.
Set broiler to low.
Brush half of the sauce over brisket and broil brisket for 4 to 5 minutes. Remove from broiler, thinly slice and serve with the remaining BBQ sauce.