Hi!
Sorry about what you're going through, low SAC marks is very discouraging especially when you're putting so much effort. Here are some thoughts that come to mind.
Don't get too caught up with the percentage, especially if everyone else is in the same boat. I know this will be difficult but it's the rank that matters and it seems like your rank is pretty good. Every single school gives out different SACs and each one will vary in structure and in difficulty so of course, VCAA is not going to look at percentages as the playing field is not equal.
In terms of the SACs itself, I recommend having a good look at the practical you did. Think about the limitations, the errors. Write up a conclusion, a hypothesis, some discussion. Treat it like you would write up a practical. So prior to these SACs you should brush up on your experimental design work. I also highly doubt that every single SAC will be a report on a practical investigation.
Secondly, it seems like the SACs you're given aren't going to be like the final exam. If you look at VCAA exams it's much more theory, calculation and application-based. DON'T stop doing what you're doing in terms of notes, checkpoints, edrolo... that will set you up for the final exam. VCAA has a distinct style of testing students and many schools do not emulate that style in their SACs, doing checkpoints will expose you to them from this early on. At the end of the day, the exam will NOT be an entire evaluation of a practical report.
I've had a similar experience last year. Half the sacs we were given weren't your standard tests, they were based on practicals and we basically had to evaluate practicals with a little bit of explanations and we were never given prior exposure or practice sacs. I did BADLY on them. Badly enough that I'd given up hope on a 40+ but I just continued to do what I needed to do. It hurts not getting the results you want, but you don't need 90-100%s to get those results in the end. You have great study habits so don't get discouraged and don't get unmotivated by the %s.