I once was in your shoes. I could've done medical science adv, science adv or pharmacy @ USYD and hoped to transfer. But I wasn't willing to take the risk as it was too great.
So I did Business (something I never did in high school) and IT (something I liked and did well in high school)... ended up with a GPA of 6 for sem 1. I'll be aiming to transfer medicine, if not.. m happy with my backup
Go with your heart. If you believe that you stand a chance through something more relevant, then go ahead and do medical science.. If you're aiming Gamsat... then science related degrees are the way to go. However, for transfer; it really doesn't matter as long as you have a reasonable gpa. I went with the harder option and did something irrelevant to the medical field.. and my medical science buddies (claiming to be doing an easier degree) are impressed with my gpa which is higher than theirs. Not knowing that I did all nighters before the exam lol. I was always a science person just like you, coming 1st in Biology and 4th in Chemistry at my school... and also came 1st in both IPT and IT. So if you think that the medical science option is too risky, find something else that you can excel at (for me it was IT).
So really think this hard. What will happen if you cannot tranfer or do not get into medicine through the adv science/medical science pathway? Will you lose hope and courage? OR Try something different that you're good at and something that serves as a great backup and still aim for medicine. If you fail to transfer, the worst that will happen is you graduating with a useful degree, unlike pharmacy/medical science/science adv... where the market is super saturated by Medicine hopefuls who most of em once aspired to do medicine/dentistry. My cousin graduated with medical science degree with a D average and she is jobless. There are a lot of pathways and options... doing a science related degree is advantageous and prepares you for Gamsat and also lowers the Umat and GPA requirement for some uni's; but is it a risk that you're willing to take? The UWS scheme only accepts 6 students under their medical science adv/science adv program (6 students from 3-4 campuses... not from each campus). The competition is insanely tough. Choose wisely my friend.