Line a 13-x 9-inch freezer-safe baking dish with parchment paper.
Pulse the graham crackers in a food processor until they're fine crumbs, or use 4 cups of graham cracker crumbs. Mix the crumbs and melted butter in a large bowl until combined. Press firmly into the bottom of the prepared baking dish in an even layer. Freeze until firm, about 20 minutes.
Beat the cream cheese, lemon zest, lemon juice, and 1 cup of powdered sugar in a large bowl with an electric mixer or in the bowl of your stand mixer with the paddle attachment until fluffy, about 3 minutes.
Beat the whipping cream and remaining 1/4 cup of powdered sugar on high in a separate bowl until stiff peaks form.
Fold about 1 cup of the sweetened whipped cream into the cream cheese mixture.
Spread the cream cheese mixture over the crust and freeze until set, about 1 hour.
Whisk the milk and pudding mix in a large bowl until smooth and thickened, about 2 to 4 minutes, then pop it into the fridge to chill while the cream cheese layer is freezing.
Spread the pudding over the frozen cream cheese mixture. Freeze until set, about an hour.
Gently spread the remaining whipped cream over the frozen pudding layer and freeze until firm, at least 4 hours or up to 24 hours.
Lift the parchment paper out of the baking dish and cut your Lemon Lush into squares or bars. You can serve frozen or let it thaw slightly.
Enjoy!