You can definitely get above an 80 ATAR, but you really need to step up your game for English Standard, Business, and most importantly, Maths Standard. Get some tips from @its_ace21 to improve for maths!!!
I have mates that have gotten in and although the interview process is rigorous just practice being able to deal with pressure or thinking on the spot... 👍
You're only in year 11 so relax... Also you probably haven't been consistent in studying or even know how to study, hence the "4 all nighters"... Don't drop to standard math if you want to do Comp Sci... However, because Comp Sci is oversaturated, I wouldn't recommend that major... Overall...
Apply for all, and yes Comp Sci is oversaturated so if you get into actuarial stick with it... Also, you would struggle doing a double degree in actuarial and comp sci, unless you have no social life or you're a genius...