Stir together strawberries, lemon zest, lemon juice, and 1⁄2 cup of the granulated sugar in a medium bowl. Cover; refrigerate at least 30 minutes or up to 1 hour.

Preheat oven to 425°F. Whisk together flour, baking powder, baking soda, salt, and remaining 2 tablespoons granulated sugar in a large bowl. Cut in butter with a pastry blender (or use fingertips) until mixture resembles peas. Add buttermilk, and stir gently with a spatula or wooden spoon until well combined and shaggy. Turn out dough onto a lightly floured surface, and knead gently until dough just comes together, 2 to 3 times. Pat dough with lightly floured palms to 1 1⁄2-inch thickness (do not use a rolling pin). Cut out biscuits using a 2 1⁄2-inch round cutter, gathering and patting scraps once more to make 12 biscuits.

Place biscuits about 2 inches apart on a parchment paper-lined baking sheet. Refrigerate 30 minutes. Brush tops of biscuits with 2 tablespoons of the heavy cream; sprinkle with sanding sugar. Bake in preheated oven until golden brown, 18 to 20 minutes. Cool on baking sheet 10 minutes.

Place powdered sugar in bowl of an electric stand mixer fitted with whisk attachment. Scrape vanilla bean seeds into powdered sugar; add remaining 2 cups heavy cream. Beat on medium speed until soft peaks form, 4 to 5 minutes. Split biscuits; place on plates or a large platter. Generously spread lemon curd onto bottom halves of biscuits. Spoon blackberry mixture on top of lemon curd. Dollop with whipped cream and sprinkle with basil. Cover with biscuit tops.