depends on goals, if ur aiming for a 90 wam it is a fair bit of work but to just pass or get like credits it’s really easy. and also ur major and units
high school isn’t uni, there’s not set percentage where if you get below you’ll “fail” a course. people got like 15% in some prelims at my school and had no problems
mostly y10-12, you don’t need anything too crazy though even for early entry like as long as u can find literally anything to talk about you’ll be fine
only matters for early entry and early entry mostly only applies to courses that are easy to get into with atar anyway
for almost every course for atar entry it doesn’t matter one bit, there isn’t even a chance for you to tell them