How do I nail the right voice for a specific audience in my copy?

You can already write copy that persuades. What still takes you too many tries is voice, matching your words to how a particular group actually talks and thinks. That is a finer skill than persuasion itself, and it is slow to come because it depends less on writing and more on listening. Voice is not invented. It is borrowed from the people you are writing to. The writers who hit it fast are steeped in their audience, they have read how those people speak, the words they use, the things they care about, until the right register comes naturally. It is research wearing the mask of talent. Before your next piece, spend real time in your audience's own words, their reviews, their forums, their messages, and collect the exact phrases they use. Write with those, not at them. The more you soak in how they actually speak, the fewer tries it takes to sound like one of them.