While the development of a "freemium version" might help you build a customer base, it could consume a significant amount of your time and money without the assurance that your free users would every become paying customers.

Since you're a one person operation, and running on limited capital, I'd suggest a different approach. If indeed you've gotten good traction with big companies, I'd approach one ore more of these companies and ask them to invest in the continuing development of your product up-front, in exchange for a significant discount on the per-user licensing costs. This will both serve as validation that your application is as valuable to your target customer base as you believe it to be, and would also provide you the additional income necessary to move forward.

You might also put serious consideration into trying to find a salesperson who would work with you for either straight commission (if you expect your deals to be large enough to make this attractive), or for equity. In either case, this would enable you to significantly increase the number of paying customers you can target with your unique solution.

I'd be more than happy to discuss these and other strategies in detail with you if you'd like to schedule a call with me. Good luck!