I don't see a reason not to give Mathematics Extension 1 a try for year 11. While I dropped Mathematics Extension 1 and kept Mathematics Advanced at the end of year 11, I did this for 2 reasons:
- I did not need to take this subject to supplement my knowledge in terms of my university studies. Mathematics Advanced was largely sufficient.
- At some point, I found that the content became too difficult. Given the above reason, I thought it was not worth it to keep Mathematics Extension 1 and do my best to perform well in this subject despite its difficulty.
However, your case is obviously different to mine, especially since maths is your strong point. Because of this, it seems like you would enjoy taking Mathematics Extension 1.
While taking Mathematics Extension 1 concurrently with Physics may be beneficial in terms of your studies of Physics, it is not necessary to take a particular level of maths to ensure favourable results in Physics. Of course, with that being said, it wouldn't be bad to be able to capitalise on this slight advantage, particularly if you are good at maths.
If you find that the workload becomes overwhelming alongside the rest of your subjects, you can always drop Mathematics Extension 1 at the end of year 11.
I hope this helps!