Vegetable oil spray 2 pounds high-quality white sandwich bread, such as Arnold or Pepperidge Farm, cut into 1/2-inch cubes (16 cups) 12 ounces sweet Italian sausage, removed from its casing 4 tablespoons (1/2 stick) unsalted butter 2 onions, minced 3 celery ribs, minced 2 tablespoons minced fresh thyme or 2 teaspoons dried 2 tablespoons minced fresh sage or 2 teaspoons dried 2 1/2 cups low-sodium chicken broth, divided 2 large eggs 2 teaspoons kosher salt 1 teaspoon freshly ground black pepper

Line slow cooker with foil collar — press it into the back side of the slow-cooker insert; the food will help keep in place during cooking — and coat with vegetable oil spray. Adjust oven racks to the upper-middle and lower-middle positions and heat oven to 250°F.
Spread bread over two rimmed baking sheets and bake, shaking pans occasionally, until edges have dried but centers are slightly moist, about 45 minutes, rotating baking sheets halfway through baking. Let bread cool for 10 minutes. Transfer to a very large bowl.
Brown sausage well in 12-inch skillet over medium-high heat, breaking up large pieces with wooden spoon, about 5 minutes. Transfer to bowl with dried bread.
Add butter to sausage drippings left in the pan and melt over medium-high heat. Add onions, celery, thyme, and sage and cook until vegetables are softened and lightly browned, 8 to 10 minutes. Stir in 1/2 cup broth, scraping up any browned bits; transfer to bowl with bread.
Whisk remaining 2 cups broth, eggs, salt, and pepper together, then pour over bread mixture and toss gently to incorporate. Transfer to prepared slow cooker. Cover and cook until stuffing is heated through, about 4 hours on low.
Remove foil collar and let stuffing cool for 10 minutes before serving. (This dish can be held on warm setting for 1 to 2 hours before serving.)