But no matter what's in your contact, that route is probably not your best option. From what I understand, you created, in your own time, a project that is valuable to your employer, and that they want to use and develop. You're the expert on this project. Not just that, you designed every aspect of it, popularised it, and your employer apparently likes it enough to want it. These are valuable skills. At the very least, this should imply a nice raise. Perhaps a promotion. You're the natural PO and the natural lead developer for this project.
I would discuss this with your employer. See if they're willing to give you a nice promotion with this project as your primary responsibility and an accompanying raise. Maybe a bonus too.