In a large saucepan add the frozen berries, ¾ cup sugar, and lemon juice, stirring thoroughly, place over medium-high heat. Heat until berries thaw and began to release their juices, stirring regularly and breaking up large pieces of berries with a spoon or potato masher. Bring to a boil, reduce heat to medium to simmer and reduce slightly, 6-8 minutes.
In a small bowl stir together the tablespoon of cornstarch with one tablespoon of cold water to make a slurry. Add the cornstarch slurry to the berry mixture, stirring to incorporate and mixture begins to thicken. Remove the berry mixture from the heat, pour in a large shallow dish to cool quickly, set aside to cool completely. Berry mixture can be stored in the refrigerator up to 2 days before assembling, makes 4-5 cups of berry filling.
To the bowl of a stand mixer fitted with the paddle attachment, add the softened cream cheese, 1 ½ cups sugar, and lemon zest. Beat at medium speed for 30-40 seconds, increase speed to medium-high, continue to beat until mixture is smooth and well incorporated, 60-90 seconds. Scrape down the bowl and paddle.
Fit mixer with the whisk attachment, add the heavy cream and vanilla to the cream cheese mixture. Mix on low until the cream beings to incorporate with the cream cheese, 45-60 seconds, scrape down the bowl. Increase speed to medium-high, beat until the mixture increases in volume and forms stiff peaks, 1-2 minutes. Makes 5-6 cups of cheesecake filling.
Line the bottom of a 9x13 dish with graham crackers, cutting with a serrated knife if necessary to fit. Add ⅓ of the cheesecake mixture, about 1 ½-2 cups, spread into an even layer with an offset spatula. Add ⅓ of the berry filling, about 1 ½ cups, spreading into an even layer. Repeat layering two more times, making three layers of graham cracker, cheesecake filling, and berry filling.
Cover and refrigerate 3-4 hours until crackers have softened and fillings are set.