Is it obligatory for the wife to return to her husband for the sake of her daughter, or how can she maintain custody of her daughter after the age of seven if she does not wish to return to him?

It would have been better for you not to leave your home. My advice to you is to return to your husband, especially since there is a child between you. Try to change your approach and show him obedience, while being kind to him and supplicating for his guidance. Regarding custody, the mother has a greater right to custody as long as she does not remarry. The proof for this is the saying of the Prophet, peace and blessings be upon him: "You have a greater right to him as long as you do not marry."