I found maths extension 1 substantially harder in year 12, but that may have to do with my school as they had harder tests, I found the content quite enjoyable just the tests were crazy. I ended up dropping it as my ranks were too bad. It's good for those who enjoy maths, if you're just doing maths for the sake of doing maths, then chose advanced I reckon
Chem is pretty enjoyable, it's a nice balance between calculations and content, both of which aren't too hard. It can get hard for some people, I went tutoring for the subject which made it much better, but yeah it seems to be hit or miss, some like it some don't. I also found year 11 really hard (hence why I got a tutor) but year 12 has been pretty alright so far and I haven't needed to dedicate a substantial amount of time to it
Physics I absolutely hate, but that's because I really don't like the content and nothing clicks for me. If you are both and science and maths person, I think you'll enjoy it. Year 12 has a nice balance between content and calculations, but it got pretty complicated for me at times
I don't do biology but from what I've heard it's heaps of content that requires a lot of memorisation. Year 12 I believe has a larder focus of stuff like diseases, but I'd check the syllabus
Idk anything about legal studies lol