Cook sausage in large skillet over medium until browned and fully cooked, crumbling finely. Drain excess grease; cool slightly.
Thoroughly mix sausage, cream cheese, 1 cup shredded cheese, rice, marinara, garlic, Italian seasoning, optional pepper flakes, salt, and pepper in large bowl.
Carefully fill each pepper half with about 1-2 tablespoons filling, packing gently but firmly.
Pour about ½ cup water or broth into slow cooker bottom for moisture.
Arrange peppers in single or double layer, upright if possible; drizzle extra marinara.
Cover; cook on low 2½-3 hours until peppers tender but intact.
In final 15 minutes, add remaining ½ cup cheese; cover to melt.
Garnish with basil or parsley; serve warm with toothpicks.