Repeating year 12 is rarely recommended. You should only consider repeating year 12 under exceptional circumstances which, given the reason for which you are contemplating repeating year 12, is not something that appears to be relevant to you. There are some significant issues with repeating year 12, especially if you are not in a situation where repeating year 12 would be necessary, including:
- Wasting an additional year, which you could otherwise be using to complete your first year in a university degree (or similar).
- Having to work towards a particular ATAR goal from the very beginning, which would place you under pressure for a second time, the only difference being that the pressure from repeating year 12 is not necessary and unjustified.
Not only can you gain more in-depth/specialised knowledge in a particular area by completing a university degree (as opposed to HSC level knowledge, which is limited in comparison), but if the field that you wish to study at university is different from one (or more) of the subjects that you wish to take if you were to repeat year 12, you can always dedicate some of your free time towards approaching those subjects on your own (i.e. separately from school). Not only can you achieve the same learning outcome that way, but you can also do so at your own pace, without the pressure associated with taking the subject(s) as a year 12 student.
I hope this helps!