Hey there
The advice I would give is to drop one subject for year twelve, and only do five 3/4s in total.
I think it would be easier to spend more time working harder on five subjects, than trying to spend your time working across six subjects. Doing six subjects would only mean that you have one extra subject in your bottom two, and for such a small bump to your ATAR as that, I don't think it's worth the stress of having to do a bunch more sacs and an extra exam.
When I was in year eleven I did six subjects: four 1/2s and two 3/4s. This was very challenging. Juggling six subjects was so hard, and that was when only two of them were 3/4s. if ALL of them were 3/4s I would have been stressed beyond belief, and I'm certain I would have scored worse across the board. You might be a lot more organised than I was, but I just think for anyone it would be a struggle.
SO, if I was you I would work out which subject I like least, and drop it. But also this is just my perspective!!!! Of course if you feel you're up to doing six 3/4s at once, go for it! But i just really think it would be too massive a workload.