As was mentioned above, an informed decision requires you to first be certain that improvement in Mathematics Advanced is just not possible, which means that you should complete your final year 11 task before making a decision.
As far as I am aware, both UTS and USyd treat Mathematics Advanced as assumed knowledge (rather than a prerequisite) for the Bachelor of Design in Architecture. This gives you a bit more flexibility in the event that you are unable to improve your standard of performance in Mathematics Advanced because you can still receive an offer for this degree even if you drop to Mathematics Standard. The only problem is that it is recommended to have studied Mathematics Advanced as you mentioned. Based on this, you may wish to consider one of the following options:
- Keep Mathematics Advanced - The fact that you are in year 11 means that you are taking at least 12 units, 2 of which come from Mathematics Advanced. You could keep Mathematics Advanced going into year 12 and focus on your other 10 units which, given your circumstances, would be the ones to count towards your ATAR, negating any effect that Mathematics Advanced would have otherwise had on your ATAR. This means that you could focus most of your efforts on studying/preparing for your other subjects while doing the minimum required in Mathematics Advanced. Doing so will allow you to ensure that you meet the assumed knowledge recommendation, while also protecting your ATAR.
- Drop Mathematics Advanced - Mathematics Standard is generally considered to be easier than Mathematics Advanced, meaning that you may be able to perform better in this subject, provided you study to the best of your ability. While this option means that you would not gain the assumed knowledge for your chosen degree, it gives you the opportunity to perform well and maintain 2 backup units, as opposed to keeping Mathematics Advanced, in which case you would technically only care about your other 10 units. Following your HSC, you can complete a maths bridging course, equipping you with the appropriate knowledge for you to smoothly start your degree.
I hope this helps!