Third year actl student here!
If I were you I would seriously question whether actuarial studies is right for you...
Not necessarily due to the difficulty of the math (which of course is very high, people who get good marks in 4u may still struggle in first year math), but also because of the volume of math. You really need some sort of passion for mathematics because you will be exposed to so much of it. Would you really say you have that passion given you only chose to do general maths?
In terms of content, high school probability doesn't even come close to what you'd encounter in Actuarial Studies, it's actually quite a flaw in the HSC system that so little statistics and probability is taught (but that's another topic). The probability and statistics in actuarial studies require a very solid foundation especially in calculus, and again, there are plenty of students who have that foundation in high school but still struggle at uni.
In my opinion, it would be very hard to come into Actuarial studies without 4u, let alone 2 and 3 unit, I would strongly recommend NOT picking Actuarial. You seem to think that you are naturally decent at maths, sorry to sound rude and I don't know your story, but if you're basing that purely off success in General Mathematics, you may be in for a rude shock. Just so you get a taste of what kinda of maths you'd be faced with at uni, go onto youtube and look up "unsw math1131" and watch some videos there.
I apologise if this sounded harsh but I think you need to hear it now. Let me know if you have any other questions, I'll be glad to help!