Prepare the fruit: Cut watermelon into 1-inch cubes, place in a colander to drain, then pat completely dry with paper towels. Wash and dry berries, halving any large strawberries. Set aside.

Make the cream layer: In a large mixing bowl, beat the heavy cream with an electric mixer on medium-high speed until soft peaks form, about 2-3 minutes. Add powdered sugar and vanilla extract, then beat until stiff peaks form. Set aside in the refrigerator.

Prepare the cream cheese mixture: In a separate bowl, beat the softened cream cheese with granulated sugar until smooth and fluffy, about 2 minutes. Gently fold half of the whipped cream into the cream cheese mixture until fully combined and no streaks remain.

Begin layering: In a large trifle bowl or individual serving glasses, start with a layer of crushed vanilla wafers (about 1/3 of the total). Add half the watermelon cubes in an even layer.

Add the creamy middle: Spread half of the cream cheese mixture over the watermelon, smoothing gently with a spatula. Top with half the mixed berries, pressing them lightly into the cream.

Repeat layers: Add another layer of cookie crumbs, the remaining watermelon, the rest of the cream cheese mixture, and the remaining berries.

Finish and chill: Top with dollops or a smooth layer of the reserved plain whipped cream. Sprinkle with remaining cookie crumbs, chopped mint if using, and lemon zest.

Refrigerate for at least 2 hours before serving to allow flavors to meld and layers to set. Serve cold within 12 hours for best texture.