When is It Time to Bring in a Managing Partner?
I think that you have answered your own question about the need to add a management expert to your team. From what you have said, it seems to me you need someone sooner versus later. You are losing opportunities that may not appear again. The questions seem to be: (1) What exactly would you want the person to be in charge of? and (2) How do you go about engaging him/her?
If you have sat down and strategized where you want to go -- put together a well-thought-out business plan -- and have recorded all of the major aspects of running your business, the next step is to determine who will handle what. Then you will have at least started to define what kind of person you need to find in terms of the candidate's knowledge, experience, skills, mindset and so on.
At this point, you may feel that you have all kinds of flexibility about who does what, but that is not necessarily so. Hopefully, you know what you do best and will concentrate on finding someone who either does what you do not do well or does the things you really don't enjoy doing. In this case, if you "hire in your own image," you will have two people doing the same things or at least wanting to do the same things and no one doing the other things that may be of equal or greater importance to the success of the business.
Next, develop a job description for the managing partner position. Once that is done and you feel comfortable that you have captured the essence of the position, you can plan how you will compensate and reward this individual. If you do not want someone on payroll, you will need to formulate an equity deal or engage an independent contractor who may be working for other entities at the same time. The latter might work out fine. You can always bring the person on board and offer equity if he works out as a contractor.
I do recommend that you use a good business attorney to help you draw up your agreement with this person -- one who has done many small business partnerships and other deals in which some equity is included in the mix. If you do not, I predict that you will come to regret this oversight.