OP: My employer wants IP I've created. I want to negotiate compensation for it.
Your advice: Start by sending them a resignation letter.
Negotiations are all about relationships and starting by triggering others fears of abandonment is not the way to collaborate to a mutually beneficial option.
I would suggest that the OP asks the if the employer would be open to discussing what he wants "given the additional value I'd be providing the company, I'm wondering if we could discuss my compensation and control over the project and any interests you may have?", and seek legal advice if he's not confident asking.
It's important that the as initial ask is not phrased as anything extortionary ("give me x or I will/won't do x") as that kind of talk is more likely to trigger legal concerns.