o.O i did six subjects in my last year (also including two englishes, spesh and chem) and it wasn't the worst thing in the world.

imean yeah, it was tough, but year 12 is gonna be tough for everyone. I feel like, regardless of if youre taking 4,5 or 6 subjects, you'll get used to the workload after a little bit and it'll just feel normal.
but with that said. your sixth subject isn't going to make a huge difference to your atar, so if youre just trying to get the highest score possible, it's probably more effective to just concentrate on five. imean, i took six in my last year because i wanted to do everything. and three of the nine 3/4s i did in total didn't contribute to my atar at all. so imean, if you wanna think about it that way, yeah there wasn't that much point doing those three, and i probably would have done better if i took less subjects overall and spent more time on each of them. but at the end of the day everything turned out ay okay.
also if you like music, taking music performance gives you an excuse to play which is nice. cause i know a lot of the time in year 12, people neglect the things they enjoy because they feel like they should be studying all the time.