Hey there! Welcome to the forums!
While Trials do make up a significant portion of your overall HSC, ultimately it's more about how you perform as a cohort and your own mark in the HSC. Averaging in the 60s with a decent rank in a strong cohort that will perform come HSC is still fine. Just remember marks don't matter as much (except for perhaps self-esteem + confidence building ie. its numerical significance on you, but that seriously depends on your expectations) and it's the ranks that matter more (internally at least). The best thing you can do right now is to always make your next exam your best one yet and just back yourself.
Estimates for a raw external mark on the actual HSC I'm not too sure of, but just remember in general your Trials will be a lot easier to 'scare' you into studying harder for the actual HSC. Given your expectations and capabilities, I don't doubt you have the ability to hit high 80s and higher. I take it the only things stopping you are panic, stress, anxiety and silly mistakes? If so, the best way to combat those is to calm down before exams in whatever way suits you best. I know this seems like a bit of a cop out answer but it seriously works; for me it was chewing gum (and throughout the exam) and not talking to anyone, but everyone has a different tactic; what suits one person mightn't necessarily suit another. When studying, reduce the number of exams you're doing under timed conditions at first to stop putting stress and pressure on yourself, then gain confidence to do more. In the exam, exam technique is seriously underrated as a skill, what I find is most important are two things: a) reading time and b) pacing yourself. Loads of people just try to solve the MC in reading time, but because MC is relatively easy, I recently started reading the exam back to front to gauge what I'll have to later in the exam, enabling me to go faster. Pacing yourself is likewise important; go as quickly as possible, but not at the expense of dropping easy marks at the beginning of the exam, thus enabling you to get to the end with plenty of time to solve the harder questions at the back end. This is primarily what Trials and practice exams are for
Hope this helps