I think I found a solution in another thread on this forum. I was going about it wrong.
You need to take text from the persona you want to clone, run it through GPT to neutralize it, then create a fine-tune on the inverse, so fine-tune Neutralized → Persona.
You have one fine-tuned model for each persona. To render the persona, you take the neutralized text and run it through the fine-tuned model.