Yes you definitely should stay in year 12 and reconsider your subject choices. The biggest advantage you have at the moment is your age and time. Get good marks in relevant subjects, advanced maths and physics. If your parents are willing to pay for a PPL better off spending that money on a tutor if necessary. Don’t take on any fluffy subjects if you can avoid it. Get this foundation in place before investing significant money on pilot training. Having competitive grades will help make your application stand out from the crowd and with the entrance testing. Then consider a cadetship with QF or VA or even entry into the ADF as a pilot if you have the citizenship.
But just as important as academics, flying experience, qualifications and entry test results is a good attitude. Like any other job, your potential employer wants to see if you’re a good risk. They’ll be investing significant resources in your training and development. Just like any other job with any other employer, getting a job as a pilot is not a right. Your potential employer does not owe you a job just because you want it or have spent significant time and/or money in your own development. Be able to provide good positive reasons as to why you want the job and what qualities you bring to the position and organisation.