If you don't mind managing other people, keeping full control will probably be less frustrating in the long run. Your business has a narrow offering, so adding multiple opinions may result in more headaches.

Management is a whole separate skill, though, so be prepared to dedicate real time to managing your team (whether it's a partner or a contractor).

Another option — assuming the marketing and design is within your capabilities — is to work on the way you schedule your time, which could make the work seem less demanding and take less time. This is what I've done (I'm a developer, designer, and marketer), and after owning an agency with both partners and contractors, I've much preferred staying on my own and being a better manager for myself.

If that sounds like something that would work for you, I'd be happy to talk with you about laying out strategies to make that work. Request a call and we'll set it up.

Good luck!