How can one deal with a future wife who loves singing, TV series, and movies to encourage her to abandon them? Is it better to be harsh and definitively cut them off, or to be patient and advise gently?

There is no harm in marrying the girl mentioned if you do not find someone better than her in religious commitment. Her being a prayerful person, of good character, and from a good family constitutes a good foundation. Her habit of listening to music and watching movies will disappear by moving into a righteous environment and finding suitable alternatives. As for how to deal with her to abandon these transgressions, that depends on her nature. Some people require flexibility and gradualness, while others benefit from firmness. Before the marriage contract, treat her as a non-mahram (definition); do not be alone with her, nor speak with her except out of necessity.