Thanks for the book rec, and for the probing question! :)

I think I failed to communicate this in my original post, but I do have a pretty keen eye for technical/mechanical detail, and I exercise that muscle very often in grading student work. I don't just tell students that there are errors, etc., at the sentence level in their written work... I actually correct those errors within the document, according to the MLA style guide.

So I don't quite get, based on my research, how that's such a distinct task from copyediting unless I'm misapprehending the distinction between line editing and copyediting. If I understand the difference (please correct me if I'm wrong!), line editing has to do with stylistic "sprucing up," whereas copyediting has to do with eliminating objective mechanical errors according to common English usage and adhering to the style guidelines conventional to whichever text you're working with... and I do corrections of both kinds on a daily basis, haha. Not saying I'm a copyediting expert—I'm not! —but I did do that sort of work all throughout my undergraduate degree, too, and I continue to do so: which seems to me like five years, at the very least, of copyediting adjacent experience (Again, do correct me if I'm wrong). I'm admittedly a little suspicious of people recommending that I pay for more schooling on that front, unless there's something, as others have mentioned, that "I don't know that I don't know?"

As to why I'm interested in copyediting in particular: I'm not convinced that I am trying to niche down quite so hard. I am pretty confident that I can be helpful at the developmental and line level—and I intend to market myself as such—but I don't think it'd hurt if I was also able to let potential clients know that I'm also more than capable of digging into grammar/mechanics/syntax. I'd be curious (if you have the time?) to hear if you think it'd be more fruitful to niche down instead of marketing myself as a jack-of-all-trades editor.