Step 1: In a large mixing bowl, beat the softened cream cheese and powdered sugar together with an electric mixer until the mixture is smooth and creamy.
Step 2: In a separate medium bowl, whisk together the instant vanilla pudding mix and cold milk for about 2 minutes, or until the pudding begins to thicken slightly. Let it sit for another 2 minutes.
Step 3: Gently fold the thickened pudding mixture into the cream cheese mixture until they are thoroughly combined and smooth.
Step 4: Carefully fold in the thawed whipped topping and the well-drained crushed pineapple until no streaks of whipped topping remain.
Step 5: Gently add the sliced bananas, sliced strawberries, and mini marshmallows to the fluff mixture. Fold gently until all the ingredients are just combined, being careful not to mash the fruit.
Step 6: Transfer the banana split fluff into a large serving bowl. Cover the bowl tightly with plastic wrap and refrigerate for at least 2-4 hours, or preferably overnight, to allow the flavors to meld and the mixture to set.
Step 7: Before serving, garnish the fluff with additional fresh sliced bananas and strawberries, a sprinkle of chopped nuts, a drizzle of chocolate syrup, and a few maraschino cherries, if desired. Serve chilled.