Heat oven to 425°F; oil a rimmed sheet. Soak bread in milk 5 minutes; mash to a paste. Gently mix with chicken, egg, meatball salt, 1/4 teaspoon thyme, and pepper. Shape 24 meatballs; bake 16 to 20 minutes until centers reach 165°F.
Meanwhile, cook bacon in a wide saucepan over medium 7 to 8 minutes until browned. Retain bacon and 1 tablespoon fat; discard excess. Add mushrooms and pearl onions; cook 8 minutes until mushrooms brown.
Add tomato paste for 1 minute, then wine; simmer 5 minutes. Add broth, bay leaf, and remaining thyme; return to simmer.
Transfer hot sauce and meatballs to a 5-quart slow cooker. Cover; cook on HIGH 2 hours, gently turning once. Whisk cornstarch with slurry water; fold into sauce. Cover; cook on HIGH 15 to 20 minutes until thickened and centers remain at least 165°F.
During final 15 minutes, boil noodles in a large pot with noodle-water salt for 7 to 9 minutes or package time, until tender. Drain; toss with butter.
Discard bay leaf. Divide noodles among 6 bowls; add 4 meatballs, mushrooms, pearl onions, bacon, and wine gravy to each.