I'm kind of worried about not choosing a Science though. In the future, I might choose to do something with population genetics but I don't see myself doing anything Economics related at uni. I did Commerce in Year 9 and 10 and found it boring (not sure if Eco is anything like Commerce). Also, I'm not really into writing essays (it's actually my biggest concern for Modern).
Economics is nothing like Commerce in Year 9/10. I found the latter incredibly boring as well.
I personally didn't like essays in the HSC either, with the exception of economics. Economics essays tend to be much more logical, and they're the kind of essays where if you understand the economic concepts taught to you, then you can generally make these essays up on the spot and they'll be good.
That said, if you'd prefer minimising essays, then switching out modern history and economics for sciences would be fair.
I thought that for most Science courses you had to complete either two of Bio, Chem, Physics or Adv Maths. Also, a lot of courses say that Chem is assumed knowledge while Bio is just recommended. Should I choose Chem instead?
As Drdusk mentioned, it's recommended to do a bit of science before going into a science degree, but it's not required. For biology, I'd argue that can come into uni fresh and be just fine.
I would personally recommend HSC Chemistry purely due to the fact that chemistry is harder to self-study than biology. At USYD, one of the hardest first year units for chemistry/biology majors is a unit called molecular biology, so chemistry would definitely be useful here!
In the end, it's really up to you. Have a look at the HSC syllabi for biology and chemistry, and pick the one that's most appealing.
Aside: I'd actually recommend picking up Mathematics Extension 1, since maths does come in handy (e.g. if you're into population genetics, then at uni you can expect to see quite a bit of statistics appearing, particularly if you end up working on research projects). But for whatever reason if MX1 isn't appealing to you, you can still learn the concepts at uni so no worries.