"hsc/prelims is a different pot of history than junior school history, am i right?"
Yes you go into deeper stuff, it is very heavy in content, you write a lot of essays, but otherwise I don't know because I don't do any histories.
Do you prefer writing essays over doing maths/science problems, and are you good at essays?
Prelim Bio is very easy. Chem - if you don't hate it and are confident with all the syllabus dot points, then you'll do well.
Yeah, maths is mainly about how much practice you do, pretty much. Even the smartest people have to do heaps of it. The SC isn't THAT reflective of HSC results. A lot can change in 2 years!