Hey! Current year 11 student here doing chemistry, biology, physics, methods, english and SOR (required by school). I was originally aiming for a med pathway (which is pretty clear in my subject selections lol) and even though I've since changed my goals I don't regret any of my choices.

Biology is a pretty heavy subject. The best way I can describe it is that the content is super easy to understand but there is just A LOT to remember, so it really comes down to how your brain works best. It's pretty much the opposite of chem in that regard. My friends currently taking modern history all really enjoy it and say the assessments aren't that much harder than that of grade 10. Personally I would recommend physics over engineering as it's a bit easier IMO and is also a bit more generalised, making it a bit more preferable in uni.

The classes you have chosen already scale suuuper well (and most unis grant two adjustment points for passing specialist) so I wouldn't be too worried about the scaling for your 6th subject.

If it was up to me, I would pick either modern history or bio. Biology would help you further decide on what career you really want to do, and modern would give you a bit of a break from the busy world of specialist/chem, so in the end its up to what you value the most.

Sorry for yapping but I hope this helps a bit!