Hey!
First of all, congratulations on your amazing ATAR! That's definitely an achievement to be proud of.
I've only just graduated year 12 myself, so take this with a grain of salt, but you have a few options to take note of. First of all, it's true that the general consensus is that doing a biomed or science degree is quite limiting in terms of job options post graduation. If you are interested in scientific research, then that is a potential option for you if you choose to do a science/biomed degree, but there isn't much else. To be frank, there is quite a slim chance anyone will get into postgrad med, in terms of applicants vs places, so people who do these degrees who tend to bank all their chances on getting into postgrad often get stuck, and end up doing honours/masters.
The question you need to ask yourself is: do you find law interesting? Can you see yourself working in law in the future? If you can, then it seems like a possible option. If you find it interesting, then it will be easier to maintain a higher GPA as you will actually want to study it. The other thing to consider if you are interested in working in medical areas is that you can also choose to do physiotherapy or pharmacy at Monash, which are only 4 years instead of law/science 5 years, still be considered for postgrad med, and get to work in these areas after graduation if you don't get in.
So, to answer your questions: you're not entirely screwed, dependent on what you want to do after your bachelors, and the degree is likely more stressful than non-double degrees only because its condensing two degrees into a shorter time. But if you are passionate about science and/or law, and can see yourself working in law if postgrad med doesn't work out, then it sounds like a great option to me.