Should the husband continue to gently try to convince his wife to wear the hijab, or should he resort to a stricter approach, and is he sinful for not having chosen a veiled wife from the outset?

One should not choose a woman who does not observe hijab. It is more preferable to choose one who is righteous in her religion and character. If a marriage takes place with a woman who does not observe hijab, the marriage is valid. However, efforts must be made to oblige her to wear the legal hijab through appropriate means, and she should not be given a choice in this matter, because the husband is her guardian and is responsible for her, and she must obey him. Allah, the Almighty, says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones..." And the Prophet, peace and blessings be upon him, said: "A man is a guardian in his household and is responsible for his subjects."