I personally think that you are in a good position to consider taking Mathematics Extension 2 for three main reasons:
- You enjoy maths, meaning that Mathematics Extension 2, despite its difficulty, would be a subject that is consistent with your interests, not to mention that you are more likely to be prepared to commit to a subject that you feel genuinely interested in (which you seem to be, given your comments) than other subjects that may not appeal to you.
- Taking Mathematics Extension 2 means that you will complete 12 units in total, 2 of which come from accelerated Studies of Religion II and the remaining 10 to be completed in year 12. The advantage gained by doing so extends beyond your satisfaction of taking subjects that you enjoy, as you will have 2 backup units instead of 1, which can negate the effect that unfavourable performance in a subject could present on your ATAR, as opposed to 1 unit, which can only halve the negative effect that unfavourable performance in a subject could present on your ATAR.
- If you feel that Mathematics Extension 2 may not be for you after taking it, you may be able to drop it, which still leaves you with 1 backup unit.
You can definitely mitigate silly mistakes. This will require you to maintain focus, regularly complete relevant practice material as well as ask questions regarding any concepts that you may not be fully confident about as you progress through year 12. I doubt that making silly mistakes cannot be addressed and that they constitute a reason not to take Mathematics Extension 2.
I hope this helps!