It is not just about promoting. The person needs to be able to play the role. Which includes scoping, scheduling, distributing the work, guiding, deciding, reviewing, etc. In a startup, you need more of a leader than a people manager. So if you have person who fit this role, then you should be okay, but if just promoting for the sake of managing is not going to help.
Managing engineers is a different skill set. Is s/he interested in it? Even if they are, realize that you're likely going to be slowing down the team since even if they're an action driven leader (coding + managing), a significant % of your best engineers time will now be devoted to other, non-coding work
The smartest person may not be the best suited for being a manager and may provide way more value for you remaining an individual contributor instead. If there's an engineer who you find works well with the team, is more of a 'team lead' type, and that is also capable of scoping stories well, that person would be better suited for being a manager.