Whisk mayonnaise, sugar, vinegar, milk, lemon juice, celery seed, salt, and pepper in a large bowl until smooth.
Fold in cabbage, carrots, and onion until everything is evenly coated.
Taste and adjust sweetness or acidity with a pinch more sugar or vinegar, if needed.
Cover and chill for at least 1 hour for best flavor and texture.
Toss again before serving to redistribute the dressing.