In a large mixing bowl, combine the ground beef, breadcrumbs, milk, eggs, chopped onion, bell pepper, celery, minced garlic, 1/4 cup ketchup, Worcestershire sauce, dried thyme, dried oregano, salt, and black pepper. Use your hands or a spoon to mix everything thoroughly. Be careful not to over-mix, as this can make the meatloaf tough.
Shape the mixture into a loaf shape that will fit comfortably in your slow cooker. You can either place it directly in the slow cooker or line the slow cooker with aluminum foil for easier removal later.
In a small bowl, mix together the 1/2 cup of ketchup, brown sugar, and Dijon mustard to create the glaze. Spread the glaze evenly over the top of the meatloaf.
Cover the slow cooker with the lid and cook on the LOW setting for 4-6 hours. The exact cooking time may vary depending on your slow cooker, so it's best to check the internal temperature of the meatloaf. It should reach at least 160°F inside before serving.
Once the meatloaf is cooked through, carefully remove it from the slow cooker using the foil (if used) and let it rest for a few minutes before slicing.