Your performance in your HSC exams (externals) are not affected by factors such as your classmates, your ranks relative to your cohort or your school rank. Your Examination Mark (which is the technical term for the mark that you receive for your external exam in a particular subject) solely depends on your own performance and is subject to the process of alignment.
Where the factors you outlined are relevant/important is with respect to the Assessment Mark (which is the technical term for the mark that you receive for your school-based assessment tasks/exams). This mark is subject to the moderation process, which uses your ranks across your different subjects in addition to the Examination Marks achieved by students in your cohort. Since you mentioned that for subjects such as Community and Family Studies as well as Design and Technology, you are the only student to receive a band 6, I would assume you are ranked first in your cohort for those subjects. This is good news because as per the moderation process, the highest Assessment Mark, which contributes 50% towards your HSC mark (final mark) for a particular subject, is adjusted to equal the highest Examination Mark (which contributes the other 50%) of any student in your cohort. For example, suppose you are ranked first in CAFS, and the highest Examination Mark achieved by a student (which could also be achieved by you) is 90, you will receive 90 as your Assessment Mark. As mentioned above, your own Examination Mark depends on your own performance. Assuming you ranked first in CAFS, if you achieve the highest Examination Mark in your cohort (use 90 as an example), you will also receive the same mark as your Assessment Mark, leading to an HSC mark of 90.
Where the cohort for a particular subject becomes competitive, it is likely that the academic standard for that subject's cohort becomes higher, which is also positive because it indicates that more students can perform well in the HSC exam for that particular subject, leading to positive effects on the Assessment Marks, as per the moderation process. With that said, it is important to ensure that you are ranked favourably relative to your cohort, which is generally preferable irrespective of the subject or cohort competitiveness, allowing you to maximise your chances of receiving a good Assessment Mark. Hopefully your hard work throughout year 12 has enabled you to achieve these outcomes.
I hope this helps!