Ask HN: How did you find a CS sub field to focus on and is such focus necessary?

I just graduated college, and have been doing full stack software engineer work since high school. With that prior experience, I landed a nearly 6 figure job at a startup. I like what I do, but I find computer security far more interesting. (We had a security incident which I helped solve, that sparked a sense of meaningful engagement within me). I'm worried that I'll become a jack of all trades, master of none, if I continue to change career paths. How did you figure out what sub-discipline to focus on, and is it poor judgement to continually switch sub fields, as opposed to mastering a sub field then switching?