I've run my core business as an agency over the last few years and I have to say that it really gets in the way of trying to do your own products. The problem is that even the smallest client tends to require handholding, and the key to being a good agency is that it really is a service driven business — so it tends to be a real time sink.

Unless you have clients who don't care or don't need handholding (which is very rare) running an agency can eat up your life. The problem is that then when you do to do a product you're constantly in stop-and-restart mode.

You might want to think of having another option: Like working for a client project, banking as much money as you, and then live off of that while you do your product.