Preheat oven to 350°. Spray a 10- by 15-in. rimmed baking sheet or two 9- by 13-in. baking pans with cooking oil spray and set aside. Melt butter and set aside.
In a medium bowl, whisk 4 eggs with milk and set aside. In a large bowl, mix cornmeal, flour, granulated sugar, brown sugar, baking powder, and 1 tsp. salt. Add milk mixture to cornmeal mixture and mix just enough to combine thoroughly. Stir in melted butter. Pour batter into prepared pan(s) and bake until golden brown, about 30 minutes. Let cool until cool enough to handle, about 30 minutes. Keep oven on.
Cut cornbread into 1/2-in. pieces and spread on 2 large, rimmed baking sheets. Bake until golden brown, stirring once or twice, about 45 minutes. Let cool to room temperature; transfer to a very large bowl.
Meanwhile, in a large frying pan over medium-high heat, cook bacon until brown and crisp. Using a slotted spoon, transfer bacon to a plate or baking sheet lined with paper towels. Transfer rendered fat to a small bowl and set aside.
Put 1 tbsp. reserved bacon fat back in the same frying pan over medium-high heat. Add celery, onion, and remaining 1 tsp. salt. Cook until soft, about 5 minutes. Transfer onion and celery to a medium bowl along with bacon. Return pan to medium-high heat and add 1 more tbsp. reserved bacon fat. Add apples and cook, stirring occasionally, until well browned and soft but not falling apart, about 7 minutes. Transfer apples to bowl with onions, celery, and bacon.
Return pan to medium-high heat and add cider. Use a wooden spoon to scrape up any browned bits on the bottom of the pan. Add cream and cook until liquid is reduced by half, about 10 minutes. Meanwhile, whisk remaining 4 eggs in a medium bowl. Add 1/4 cup cider-cream mixture to eggs, whisking constantly. Add another 1/2 cup cider cream, still whisking. Whisk in remaining cider cream.
Pour egg mixture over reserved cornbread cubes and toss gently but thoroughly. Add reserved onion-bacon mixture, thyme, and pepper. Toss gently to combine well. Transfer mixture to a 9- by 13-in. baking pan, cover with aluminum foil, and bake until hot, about 45 minutes. Serve hot or warm.
Note: Nutritional analysis is per serving.