Heat oven to 425°F; oil a rimmed sheet. Mix panko and milk. Fold in pork, egg, water chestnuts, scallions, ginger, salt, and pepper just until combined. Shape 24 equal meatballs.
Bake 16 to 20 minutes until centers reach 160°F. Meanwhile, simmer broth, honey, soy sauce, vinegar, and garlic in a saucepan 2 minutes.
Transfer hot sauce and hot meatballs to a 4-quart slow cooker. Cover; cook on HIGH 2 hours, turning gently through sauce once after 1 hour.
Whisk cornstarch with slurry water; fold into sauce. Cover; cook on HIGH 15 to 20 minutes until glossy and thick enough to coat a spoon. Confirm meatball centers reach 165°F.
Turn gently to coat; serve about 3 meatballs per appetizer portion. Hold on WARM only while food stays at least 140°F; keep covered between servings.