One of the other hats you need to occassionaly wear on a solo project is "product manager". This hat is responsible for gathering, refining, and prioritizing customer requirements. This hat has a major vote in "what features do we build next" decisions, and is responsible for keeping the customer in the loop to verify that what's being built has business value to the customer.

Projects that lack this hat (or don't fill it effectively), risk spending a lot of time and effort solving the wrong problem.

You can wear this hat if you're on a solo project, but it's tricky. The great trap is that you'll hear the first half (or third, or two sentences) of what the customer is asking for, and immediately swap in your "designer" hat and start thinking solutions. This is deadly, since a customer almost never gives a complete story on first presentation. Locking on to the wrong target can lead to a lot of wasted effort.