Just write a clear and concise summary of a contract.

“You get: [these features]”

“I get: [money and the IP]”

“We measure success and payment milestones are triggered by: [Clear KPIs and deadlines]”

Make sure the 1-2 page doc has your name on it, and ask your boss for permission to drop a paper version on the desks of the higher ups.

You can be employed and enter into a separate contract.

Edit: I just re-read the original post and saw that the OP is a software engineer. Ignore everything I said. This sounds like a terrible idea. You’ll burn all your bridges if you pursue this avenue.