I agree that you want to do some thinking before launching an external API. A nice intermediate low risk step would be to launch the API internally and see how useful it is for folks in the organization. I did that a few years ago and it turned out to be very useful.

Plus it gives you a safe space to learn about the intricacies of running an API before you charge for it.

Also, as a developer if I heard "we are exposing an internal API and charging for it" I'd be confident that the API would be around.