You mentioned that people don't take you seriously as a one-man operation. This is not my experience when it comes to negotiation of standard contracts - businesses care about the value you create for them. Being a one-man operation has mattered when dealing with acquisition negotiations, but I think optimizing for that now is a bit of cart-before-horse.
Imagine you are pitching to your imaginary board or a potential investor. You want to bring on a cofounder to help you grow X% in Y months by adding Z strategic value. Fill in the blanks and then decide if you actually know anyone like that. When you do all that I think the % of the company to give away questions will become transparent.