While u are correct about all that, but i don't think this would rly apply to op's situation. I'm not trying to discredit ur achievements or anything (a B6 in Maths ADV is amazing btw). Its js that aside from maths and english adv, the other subjects scale quite poorly so most of what I have said is going off that fact.
im not referring to OP when i say that, i was just responding to your advice to consider how since last year's exam was hard this year exams might be hard, and this being a deterrent to picking certain subjects.
i get that you are trying to tell them to be aware of it and your advice is def valuable for someone aiming for a 99+ atar, but focusing too much on scaling and the alignment, break even point etc. when deciding subjects usually deters ppl from taking subjects they are good at. it pressures people to just pick the high scaling subjects instead of picking the subject that are right for them and worrying about things that are out of their control like whether or not the hsc exam is going to be hard.
i personally know of people with 5 or more units of humanities and no maths that got a 99+, and one of them was dux of a top 50 school a few years ago. yes their subjects were low scaling but they were this person's strength and by playing to their strengths they were able to achieve a high atar regardless. so instead of picking high or low scaling subjects this person should really just play to their strengths.