I have a current specialism within a fairly specialised field, but this leaves me plenty of bandwidth to move within the field in the short term.
My background is loosely related but has given me a set of core skills which I can draw on where I see other people being employed as specialists. I try to maintain my core skills in other areas to the extent that I can commoditise them for my own purposes if needed, even though I have no interest in actually pursuing 'excellence' in them.
When I first started down my specialism a number of years ago my expectation was that my previous background would become irrelevant as it felt like quite a big pivot. I was (and remain) surprised at just how wrong I was - having additional background skills that aren't directly related makes me more valuable.
Later on you can start mixing things. For instance, I do freelance tutoring on these subjects, I use my webapp backend Python code on an Android app I have, etc..
Do both, as people always have. But start out a generalist to get an understanding of what is good to specialize in. Then pick 2, 3, 4 diverse areas to home in on.