I hope this works out for you! I am not sure how I could contribute much insight without quite a bit more detail, this strikes me as "Should I hire a lawyer?"

I hope this works out for you! I am not sure how I could contribute much insight without quite a bit more detail, this strikes me as "Should I hire a lawyer?" to which I'd say "yes." Without knowing how your partner is inclined to act or respond to negotiations, or what your negotiation strategy is and how likely it'll be effective in this dynamic it's all a giant question mark. Essentially it's like asking a website "How do I win a fistfight?" except unlike actual fistfights, there's an entire profession and industry dedicated to people that can (hopefully) act effectively as your proxy. If you're genuinely dead-set on not getting a lawyer, then you've got a fistfight scenario on your hands. If that's what you're looking for, make sure that you've taken stock of your capabilities and those of your adversary and form a strategy accordingly. This answer is basically as vague as the question though.