I think it's probably worth noting that, in high school, subjects get conceptually harder each year. HSFY, like medicine, is by and large NOT particularly conceptually difficult. If anything, most of HSFY is a step down from level 3 in terms of conceptual difficulty.
The difficulty lies in memorizing the sheer volume of information, not so much in being able to understand it. So it isn't like you have to have done all of the HSFY subjects at level 3 in order to move to 'the next level' at uni.
Year 13 is your last chance for quite some time to pick interest subjects. If you're interested in classics, take classics.