Bring a large pot of water to a boil; cook broccoli until bright green, about 1 minute. Transfer to a bowl of ice water; drain.
Combine broccoli, garbanzo beans, carrots, raisins, and 2 tablespoons sunflower seeds in a bowl. Whisk mayonnaise, sugar, vinegar, red pepper flakes, salt, and black pepper in a separate bowl. Stir dressing into broccoli mixture. Cover and refrigerate for at least 1 hour for flavors to blend. Garnish with about 2 tablespoons sunflower seeds.
