Whisk the pudding mix with cold milk in a large bowl until thick and smooth, about 2 minutes.
Whisk in sweetened condensed milk, vanilla, and a small pinch of salt until silky.
Fold in the thawed whipped topping with a spatula until the mixture looks light and fluffy.
Toss banana slices with lemon juice in a separate bowl to slow browning.
Fold in mini marshmallows and about two-thirds of the crushed wafers.
Gently fold in the bananas, keeping the slices intact.
Spoon into a serving dish and sprinkle with the remaining wafers. Chill at least 1 hour before serving.