Mix ground beef, sweet potato, onion, and oats in a large mixing bowl.

Beat eggs, salt, thyme, and pepper together in a small bowl. Add 3/4 cup ketchup, vegetable juice, Worcestershire sauce, 1 tablespoon prepared mustard, and mustard powder to the egg mixture; stir. Pour the egg mixture into the large mixing bowl; use your hands to mix together. Cover bowl with plastic wrap and refrigerate 8 hours to overnight.

Preheat oven to 350 degrees F (175 degrees C). Grease 2 8 1/2x4x3-inch glass bread pans.

Divide meatloaf mixture into prepared bread pans.

Bake meat loaves on center rack in preheated oven for 1 hour.

Stir 3/4 cup ketchup, brown sugar, and 1 teaspoon prepared mustard together in a bowl; spread over the meatloaf.

Divide ketchup mixture and spread over the loaves; continue baking for 15 minutes.