In a large bowl, combine the cornbread mix, eggs, and milk. Whisk until the batter is smooth.
Grease a 9 x 13-inch baking dish with nonstick spray. Transfer the batter to the prepared baking dish.
Bake the cornbread for 20–25 minutes, or until golden brown and a toothpick inserted in the center comes out clean. Remove from the oven and let rest until cool enough to handle, about 30 minutes.
Remove the cornbread from the baking dish and crumble by hand into a large bowl.
Quarter the scallops and add to a medium bowl. Cut the shrimp into thirds and add to the same bowl.
Add the scallops and shrimp to the crumbled cornbread, along with the crab meat, chicken stock, eggs, garlic powder, onion powder, poultry seasoning, dried thyme, salt, pepper, and 1 teaspoon fresh thyme. Mix until combined.
Grease the dish used to bake the cornbread with more nonstick spray, then spread the stuffing in the dish in an even layer. Sprinkle the top with the remaining teaspoon of fresh thyme.
Bake until the stuffing is set and firm to the touch, 40–50 minutes.