I don't think they will be in a strong negotiating position. In my opinion, Harry and Meghan will not be able to sustain their lifestyle. I've seen estimates that his security alone costs $5 million a year. Even if the true number is half that, I don't see how they can sustain it. They fly around in private jets and have a lot of housing costs (mortgage, taxes, maintenance). His deals with Netflix, Penguin and Spotify are profitable, but there are many expenses, including taxes, employees, production costs, etc.
I think there is enough interest for one more blockbuster book, but I also believe in the law of diminishing returns. Objectively, they have not demonstrated sufficient talent to develop lucrative careers without relying on his connections. There are rumors that Meghan is considering running for public office but even if she were successful, she would not be able to make serious money until she left office.
I think Charles would give them a home in the UK and a comfortable allowance. If they demand more than that, I think Charles and William will say no and weather whatever mud is sling at them.