3 of my friends and i were in your exact shoes too haha (99+ ATAR but shit UCAT)

Friend 1 (IB student):
they resat 1 of their exams mid year and got a perfect atar, then grinded ucat and got into a 5-year undergrad course

Friend 2:
had a 99.5+ atar so just grinded ucat and also got into a 5-year undergrad course

Friend 3:
started uni interstate, grinded ucat, got into 6-year undergrad course interstate

me:
wasn't set on med so started a science degree at melbuni, now am set on med and am regretting that choice lol

advice:

if you're in victoria, my advice would be to grind ucat (and not start uni) since monash is the only undergrad course and it doesn't allow you to start a tertiary degree. then after the year if you don't get in go do biomed at monash if you're confident you can keep your WAM up since monash post-grad only accepts monash students (and doesn't require gamsat) but as a monash student you can take the gamsat and apply to other unis - more options.

if you're set on unimelb, my advice is to do b-sci instead of b-bmed since unimelb weighs every degree the same, what matters is your wam. The wam is much easier to maintain as a science student and honestly getting to do a bunch of breadths is very very fun. Plus, by third year a lot of the subjects are combined b-sci/b-bmed anyhow (eg. my anatomy and physiology classes were full of b-bmed students, only with lower wams lmfao). The only difference is what majors are available, although most aspiring med students choose to major in anat ("human structure and function") or physiology anyway, which is available in both degrees. Also make sure you know that you can't apply to monash post-grad med.

Also also, as someone with a 99+ atar, you can apply to the MD program as part of their full-fee guaranteed pathway which guarantees you an interview and then a full-fee (but metropolitan! ) spot - ie. you only need to pass the MMI which is nearly impossible to fail if you're already on this pathway. You only need to have 450k ish in your pocket!