I didn't understand this part.
> development of my product
You are an employee. It is not your product. The company owns the product. (Have you signed any paperwork?)
> but this isn't in writing
A verbal contract isn’t worth the paper it’s written on. (They are actually enforceable sometimes, but it is much more difficult than a written contract.)
As PragmaticPulp says in a sibling comment, prepare your resume today and start applying.
It seems like having a concrete counter-offer would strengthen my position and make leaving a much more comfortable descision.
A counter-offer from another company is a prerequisite for leaving. Even a low-paying job is better than no job.
That said, don't get caught in the trap of waiting around for the perfect counter-offer. If you want to negotiate a higher salary, it's not helpful to delay months and months to find the perfect other job. If you're not getting counter-offers soon, you can skip straight to negotiating. Don't let yourself get stuck in limbo forever.