I'm currently a qualified secondary teacher and teaching at the moment.
I wasn't that great at maths in high school, but managed to find my feet and build my knowledge from essentially nothing during my bachelor's degree. I now teach Maths and Computing. You have time to fix any subject area you aren't that competent in.
Tutoring is actually different to teaching so that can't really be used as a comparison. Perhaps VCE teaching & tutoring maybe, but overall they are very different. As a qualified teacher you cannot isolate yourself specifically to Chemistry (in VCE terms). You may be called upon to pretty much teach anything - this profession is one where you are required to wear so many hats it isn't funny. Like me, I have a degree in IT but I couldn't just walk in and say "I want to only teach VCE Computing". It doesn't work like that. That's all i'll say on that matter...
however I've already come to the realisation that no one's going to hire me purely for chemistry, so I'm probably going to have to teach the other sciences as well if I want to become a teacher.
Yes. If you want to teach Chemistry you'll most likely have to also be qualified in General Science (Year 7-10) or Maths (7-12). They are the two most common pairings with a specific science method area.
You don't have to take on something like physics. Like above, it can be a different method area. Your teaching method areas for a teaching degree (if you're doing postgrad), stem from what you've studied during your bachelor degree. So if your degree is full of maths and IT subjects, then obviously your methods form that. If you do a double-major, by default these two become your method areas to teach in. If you go into a bachelor of education/teaching, you learn content as well as the actual ed component of it.... so that decision is yours.
I remember growing up that she wasn't a bright student in maths, and even in her vce she only did further maths (in which she scored somewhere around 20-25).
Actually in a very similar position - I only did Further Maths myself, scored around the 30s but now can competently teach it. The experiences beyond high school definitely helped. From a teaching pov, it actually is much different than from the student pov... the responsibility you have often motivates you to become better.
Hope that helps. Happy to answer any Q's you may have, given my relevant experience in this space.
Cheers.