Startup CTOs or founding developers are the first technical people in the business. It is natural to think your job is to write code and build software. This is backwards.
Your first job is not to build software. Your role is to use your technical expertise to help the startup figure out fast if you have a valid solution to a compelling problem, and then a valid product for a big enough market.
When I created Sol Trader between 2012 and 2016, I made the game I always wanted to play. I released early, determined to get feedback. I waved away much of the early feedback because they had not understood what I was making. The product was not finished yet. I knew what I wanted to build.
I used KISSmetrics to built in lots of analytics and events so I could track what players were doing. These metrics told me that players would play for a while, and get bored. This did not matter. I was missing key features. Once those were in place, the players would come back. I knew what I wanted to build.
I spent thousands of hours building a carefully designed, well architected game complete with a custom C++ engine that compiled in a few seconds. It was glorious.