Well, that does paint the picture a bit more clearly. By implication, you're saying that "marketing thru engineering" and "advocacy strategies" are a higher priority than converting users to paid customers.
That would imply one or both of those activities will lead to sustainable cash flow or an investment that substitutes for that.
I can't help but suspect converting users to paid customers may be a bit more important than one or both of the above.
What I'm suggesting is NOT that you change roles, but rather you act the role until you understand it...then hiring the right person is easy. Hiring someone to do something you do not understand is a good way to find the wrong person and that's very expensive for a startup at your stage.
Select three users plus ALL of your Premium users (take no more than 30-60 minutes to do this...no stalling!), call them or contact them to set up a phone conversation (face to face preferred).
When you meet with them, ask them:
How they heard of you?
Why did they sign up?
What problem were they trying to solve?
When was the last time they used it?
Why not more often?
Why not a premium user? Or why converted to Premium?
What would it /did it take to be a premium user?
When will they use it next?
Do you mind if I get back to you if I have any other questions?
Call the next one.
Discuss your findings with your co-founder.
Decide who (what type of user) you want to talk to next
I guarantee you that before you get to 15, you will know a ton more about your business model, what features are most important to work on, what type of sales person to hire (a recent college grad may do the trick...you'll find out with your calls), what better ways there are to "market through engineering" and what better "advocacy strategies" to pursue....or you will find out you need to talk to a few more because none of these users are going to lead to cash flow and help you keep the lights on.
Or maybe you find that you need LOTS more development for an essential feature to convert to Premium, and you use this data to attract an investor.