Whilst you need to be more of a generalist in a startup, you also need greater depth of experience. Having a small amount of experience in many different areas

Whilst you need to be more of a generalist in a startup, you also need greater depth of experience. Having a small amount of experience in many different areas is unlikely to get you a job in either area. Better to be good at something you enjoy and then broaden your skill base into other areas. Based on your question, if you already have some front-end skills and JavaScript knowledge, then you might find NodeJS a good place to start with the backend. At least you don't have to learn a new programming language and its best practices.