Repeat. Same thing happened to peeps who accelerated my grade - they wanted band 6s and fell a bit short.
They repeated after i explained to them that going though it a 2nd time will be a breeze. And when i mean breeze, i mean absolutely breeze. One HSC/trial paper every 2/3/4 weeks until up to the HSC will be more than enough to revise/solidify your knowledge.
The peeps that repeated didn't study until before exams and they ALL got band 6s. Unless you're an idiot, you'll get a band 6 if you repeat.
If you're doing 3u, you'll find 2u even easier and will probably not need to do any 2U until a few weeks before exams.
If your school is stupid and makes you go to 2u classes, just sit and the back and do homework of other subjects. I do hope your school is sane enough though not to make you do that and you'll just sit in a random 2U class when it's exam time
Seriously, repeat as 12 units isn't much work anyway especially if 2 of those units are units you're repeating.
Of course if you plan to do 4U and are going to stick to 4U no matter not, there's no need to repeat
If you don't plan to take my advice, so be it. But you'll always wonder whether or not you would've gotten any higher if you did take it