How do I get a “niche”. I would have loved to work in some particular field I was interested in, but my whole career I was never qualified for that because well

How do I get a “niche”. I would have loved to work in some particular field I was interested in, but my whole career I was never qualified for that because well, they wanted someone already established in that niche. So I took what I had, which were generalist jobs. Never had a problem quickly picking up relevant parts of the business domain, but never really became an expert in anything particular. I’m an expert in figuring shit out I guess, and I’ve done some more interesting things on the side, but it never mattered.