Biology is a great VCE subject.
Look, every subject in VCE is going to end up annoying the hell out of you at one point or another. Biology has a bit to remember, yes, but tbh it was pretty easy to "remember" because a lot of it was not based on memorisation (except for definitions) but good understanding of the steps/ processes involved. If you know that, you should get a good score for "explain" and application questions, keeping in mind I tended to go a little over board in my answers for safety since biology marking can be a bit particular (which also may annoy you). I would say its easy compared to other subjects, but I did it in year 11, and it was my only priority pretty much, so I didn't feel overwhelmed. But, in year 12 I can imagine the 10 sacs being a bit overwhelming. But constant study should help. And given you already did so well in your two year subjects this year, and you have an interest in medicine, I'm sure you'll do well.
To get an over 40, you can probably guess; Study, know the content, and know how to answer questions well.
In terms of not doing 1/2, you should be fine. Just perhaps revise the content relevant to unit 3/4 over these holidays. I can PM you the relevant content if you want.

Feel free to PM me if you have other questions.