This post doesn't aim to be THE solution in regards to story splitting, but an illustration of ONE of the way to do it, used by a team that found it useful.
While the individual practices of TDD, DDD, and BDD are all valuable in their own right, it is the point at which they intersect that will provide the real value moving into the future.
Being a Scrum Master in a startup environment comes with its fair share of challenges, but it also offers a unique opportunity to really make a difference.