For very specialized subjects, I would completely agree. But when it comes to lower-level math - a group of topics that surely have been done to death countles

For very specialized subjects, I would completely agree. But when it comes to lower-level math - a group of topics that surely have been done to death countless times over, there really should be well-known books (or online courses) by now that learners don't get stuck on. If there aren't, that begs the question why. Though now I'm getting into pointless theorizing territory - as far as practical advice goes, it's true that having regular in-person lessons can also help a lot with discipline - so that's definitely a plus compared to self-learning. On the other hand, just like with identifying quality materials, you need to be able to identify a quality teacher.