This.
OP, you should see your current situation as practice. Once you get to uni, you might find that the teaching styles (or, god forbid, the competence) of your lecturers and tutors to be insufficient for your learning. In that case, often you have no choice but to stick with the course in order to fulfill the requirements of your degree/major, and so you have to learn everything yourself through notes, textbooks and online. Since the HSC is much easier than uni and with way more resources available online and offline, you should definitely put in the effort yourself.
And it's definitely possible to do amazingly well in a subject from self-studying only. The dux of my HSC cohort essentially self-studied the entire extension 2 maths course with the textbook our class uses (with no external tutoring), and performed fantastically.
Also, like you mentioned, chemistry is good to know for your uni course in the future, and that alone is probably enough of an argument for most people to stick with your class.