Preheat the oven to 325 degrees F (165 degrees C). Grease and line a 9x13-inch baking pan.

Combine crushed pretzels, brown sugar, and milk powder in a medium mixing bowl. Add butter to the bowl and toss to combine. Press the pretzel mixture into the bottom of the prepared pan.

Bake in the preheated oven until lightly browned, 12 to 15 minutes. Transfer to a wire rack to cool. Increase oven temperature to 350 degrees F (175 degrees C).

Whisk sugar, eggs, flour, and grapefruit zest together in a large bowl. Whisk in orange juice, grapefruit juice, orange aperitif, gin, and vermouth. Pour over the pretzel crust.

Bake in the preheated oven until filling is set, 30 to 35 minutes. Cover with foil if filling starts browning too quickly. Cool completely on a wire rack, 20 to 30 minutes. Refrigerate until firm, about 1 hour.

Pulse freeze-dried strawberries in a food processor until finely ground. Use a fine-mesh strainer to dust strawberry powder over the filling. Cut into 24 bars and serve.