Only doing what you're good at seems at odd with running a start-up, where almost by definition you need to fight through stuff you don't like or excel in.

If you're persistent you will eventually get to "design" your own job, but that's some years down the road. If you want to concentrate on the things you are good at today, look for a nice 9-to-5 job at a company that matches your values and don't look back.