Is it permissible to continue gradually reducing sins—like masturbation—with the intention of eliminating them, or must one stop them completely and immediately?

Repentance is immediately obligatory and must not be delayed. It applies to all sins, whether minor or major. Its pillars are: abandoning the sin, regretting having committed it, and resolving never to return to it. What you mentioned is not considered true repentance, but reducing a sin is better than committing it excessively. You must perform sincere repentance from masturbation by immediately abandoning it, regretting what has passed, and resolving never to return to it.