@Super_Me What might help here is thinking about why you want to do a PhD in Sociology. Is it because you love sociology right now and that it draws you so much more than everything else? Or is it more that it sounds like a cool thing? Or something else?
Nothing here is a wrong answer, but I'm just wondering about where your interest and love in/of Sociology sits in comparison to any of the other things you want to explore. Because in an arts degree it's really about balancing what currently seems interesting to try, and what you want to ultimately get out of the degree. If what you love is sociology and what you want to get out of it is expertise in sociology, then the advice of taking more sociology units may really be something to look at.
If it's more that you want to get a bunch of varied knowledge out of it, potentially be an academic but maybe not necessarily focusing in sociology (perhaps you haven't found the thing you love yet?) or something else entirely then having a more varied degree may be better for you.
In saying all of this though, take a look at your course rules/requirements, because (at least where I am) as long as you have your major (and perhaps minor) you should be fine to diversify everything else, and if you've satisfied that for sociology you'd assume that it would be enough to qualify for furthering it as your thing. Also any info on further study you can find, as, again in my experience/where I am you don't necessarily need to have a ton of units or have even majored in the area you go on to focus your academia on. I have lecturers and tutors who have different focuses in different degrees or only realised what they really want to be in after their PhD was done, (in fact I'm pretty sure one of the main sociology people at my uni didn't come across it until quite late in her career as it wasn't really a thing when she studied). With all of this simply speaking to a course advisor may be best/easiest as they will be able to give you a more solid answer and do actually know for sure.
Also, if you look through the units in different disciplines they'll often actually be covering the same or similar stuff, it's just looking at it from a different angle. I want to take advantage of every discipline and unit that slightly interests me too, but trying to figure out what units are probably pretty similar and what way I'd rather approach that content has helped a lot. And I've made a bit of a resolve to continue studying somehow regardless of what my course does. I can always do another course or do some single units. Your knowledge and study and units are not confined to the amount of units this degree has.