You can do graduate medicine regardless of whether you studied biomedicine or pharmacy. I'm not sure how you see pharmacy as a safer option, they're two different career pathways. If you have an interest in pharmacy but you're not sure whether you would also like to study medicine at some point in the future, then studying pharmacy and deciding after the end of first year (a transfer) or at the end of your degree (applying for P/G medicine) sounds like the best option here. Bear in mind, you don't need to do prereq subjects if you accept an offer to Monash or Deakin, which are the other two Victorian PG med courses.
The UMAT has no bearing on how "risky" it will be for you to apply for postgraduate medicine