You are correct. If you take 12 units worth of subjects in year 12, only your best 10 units will count towards your ATAR (consisting of your best 2 units of English and your best 8 units across your other subjects). I believe there are a couple of perspectives in relation to your question:
Keep Mathematics Advanced or take Mathematics Standard - 12 units:
Keeping Mathematics Advanced or taking Mathematics Standard in year 12 means that you will have 2 backup units, which may be useful in the event that your performance in another subject is unexpectedly unfavourable. In such a case, provided your performance in Mathematics Advanced or Mathematics Standard is sufficiently good, it can count towards your ATAR instead of the other subject, thus avoiding the negative effect that unfavourable performance in that particular subject would have presented on your ATAR, had it counted.
The issue here appears to be relevant to your performance potential. If you are certain that you cannot improve in Mathematics Advanced, then keeping it in year 12 may result in added workload that is unnecessary, when instead you could be focusing on the rest of your subjects. An alternative would be to take Mathematics Standard, which is easier in nature compared to Mathematics Advanced and may provide you with an opportunity to achieve your academic goals.
However, if you believe you can improve your performance in Mathematics Advanced, then keeping it may be beneficial for the reason I mentioned above. If not, you could switch to Mathematics Standard, preserving the 12 unit advantage. Of course, the best strategy would be to keep Mathematics Advanced (or take Mathematics Standard) as you commence year 12 and see how you go. If you are able to significantly improve your performance, you should probably keep it in year 12. If not, then at that point you will have obtained all necessary information that would allow you to make an informed decision, which in that case would be to drop Mathematics Advanced or Mathematics Standard.
Drop Mathematics Advanced - 10 units:
Dropping Mathematics Advanced in year 12 means that you will not have any backup units. In such a case, all your subjects will count towards your ATAR and the negative effect presented by unfavourable performance in a particular subject cannot be mitigated due to the lack of backup units. However, several students who kept 10 units worth of subjects in year 12 were able to perform well across their respective subjects. This means that, as long as you can maintain favourable performance across your subjects throughout year 12, 10 units would be sufficient, and backup units would not be needed. There is also the benefit of a lighter workload in the case of 10 units, as opposed to the heavier workload that would be imposed by 12 units.
If you are certain that you cannot improve your performance in Mathematics Advanced and would not rather take Mathematics Standard, and are fully confident in your ability to perform favourably in the rest of your subjects as you begin and progress through year 12, you may wish to consider dropping Mathematics Advanced.
I hope this helps!
