Nice post, tnx for sharing this stuff and your mistakes.
Did you actually launch something in the end, or did you give up before launching the product? Can you provide the link to the app. I'm curious because I think there is a market there, in general time tracking tools are really bad for some reason. Harvest is the only one that is ok for me.
Also, time tracking is something that can be used by any agency not just freelancers.