I could have gotten a mystery mark and still gone into my course (since I had unconditional early entry). That being said, I don't regret the extra study at all. I'm just going to share a few things that might help motivate you:
1. Having a higher ATAR will keep your options open - If you decide later for some reason that you really want to do teaching at USYD or UNSW, rather than wherever your currently looking at, the minimum requirement is 80, not 60. You might also find later on that teaching is not be the right fit, and you instead want to study something else. Assuming Mathematics is definitely something you're into, you might end up interested in a Bachelor of Advanced Mathematics or something instead, which has a selection rank requirement of 93 at UNSW.
2. Having a higher ATAR will give you more scholarship opportunities at your chosen university. For example, I received an extra $5000 studying at UNSW because I was the dux of my cohort. I was similarly offered $30,000 to attend UTS, which for some stupid reason I didn't take. Getting a Co-op scholarship in certain courses like CS at UNSW drastically improves ones likelihood of finding a great job quickly after university and is a unique networking experience. Some courses also offer unique access to classes or special programs for very academically capable students, for example Dalyell Scholars at USYD for those with an ATAR over 98, and the Talented Student MATH1141 classes at UNSW.
3. Having a higher ATAR will give you confidence and prevent imposter syndrome. When you go to university, it feels like everyone (barring your group project teammates) is smarter than you, and having achieved good results previously in your HSC makes you feel more like you deserve to be at this course and your capable of performing well in it.
4. Having a higher ATAR (or more accurately, higher marks in your relevant HSC subjects), will make Uni easier. Students who performed well in 4 unit maths often find the adjustment to Uni maths super chill and far less stressful. The skills you develop in HSC are very valuable in your courses, and from my anecdotal experience, the students who struggled at the beginning of Uni maths tended to struggle throughout, whereas those who did well to begin with (because they had a lot of past HSC experience with the content) were able to keep the momentum going.
5. Having a higher ATAR might make your parents proud.