Yeah, science at monash is probably your best bet all things considered.
You can take MTH1010 which is the methods equivalent at monash if you really wanted to and transfer to science at melbourne or biomed at monash or something. You could probably transfer after half a year, depending on how well your grades are.
Another thing which has already been stated but which id like to drill in is, you don't need to do a science degree to get into medicine.
You could get into medicine with an arts degree, all you need to do is satisfy the GAMSAT subjects to be able to do well (1st year bio & chem, VCE or equivalent Physics). You could do these in electives or breadth if you really wanted to. That'll get you into deakin or monash which are fine univeristies.
Melbourne has another whole set of subjects it wants in addition to the gamsat so it might be a bit harder to do those outside of a science degree but still my point stands.
Also, remember it isn't just doing well in the gamsat, you need a high GPA as well if you think youll get a higher GPA in a course that isnt science and still have room to do the gamsat subjects i say go for it.
Remember too that 2/3s of the umatGAMSAT(oops..accidentally wrote umat..thanks zien) is non science stuff, writing and comprehension and all that.