Provided that you are ranked at the top of your cohort across your subjects (which you appear to be from your post edit), and provided that you perform favourably in your HSC exams, the effect that the performance of your cohort will have on your overall performance will be minimal (i.e. little to none).
Recall the moderation process. The highest Assessment Mark (the mark that reflects your internal performance and contributes 50% of your overall HSC mark in a particular subject) is adjusted to equal the highest Examination Mark (HSC exam mark, which contributes the remaining 50% of your overall HSC mark in that subject) achieved by any student in your cohort. If you are ranked first in one or more of your subjects (which is reasonable to assume given your own performance, as well as your school rank and past cohort performance), then this process will work in your favour, as long as you perform at a similar standard in your HSC exams. If you also achieve the highest Examination Mark in your cohort in one or more subjects, that mark will simply become your HSC mark in the subject(s).
This leaves us with scaling. This is where, in addition to the goal of maximising your ranks (which you would have been working on throughout year 12), the actual standard at which you perform becomes important. From this point, the better your performance, the higher your ATAR will be. If you take a low-scaling subject, favourable performance will essentially minimise the effect that scaling presents on your ATAR.
I hope this helps!
