Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan. Beat 1/2 cup softened butter with 1/2 cup white sugar and sucralose sweetener in a bowl until creamy. Beat in eggs, applesauce, vanilla extract, 1/2 teaspoon cinnamon, nutmeg, and allspice until smooth. Mix 2 cups flour, baking soda, and salt together in a separate bowl; stir into butter mixture, alternating with buttermilk until just combined. Fold in chopped and shredded apples and walnuts. Pour batter into the prepared loaf pan. Mix 1/3 cup flour, rolled oats, 2 tablespoons white sugar, brown sugar, and 1/2 teaspoon cinnamon together in a bowl; cut in 3 tablespoons cold butter until mixture is crumbly. Sprinkle topping over batter. Bake in the preheated oven until a toothpick inserted in the center of the bread comes out clean, 55 to 60 minutes. Cool in pan for 10 minutes before removing to a wire rack to cool completely.