I recommend at least trying 2 Unit Mathematics. Parts of it can be challenging, but it lays the foundations required for quite a few areas of Commerce. Calculus is one of the greatest tools provided by mathematics, and without that knowledge (and among other areas of basic mathematics), you will be significantly hindered compared with your university cohort.
I feel as if while you may be applying effort, you aren't apply it correctly. If you are enrolled into 5.3 maths, then chances are you are capable of managing it. Hit up some past papers, and/or see if you can book a few lunch times with your teacher. Maths is one of those subjects in school where you can master it simply by doing as much of it as possible (e.g. try aiming for minimum 3 hours each week).
I'm straight up suggesting 2 unit over general because well 1. I don't know the syllabus, and 2. it has calculus. General maths also scales horrendously; E.g. to hit your aim of an ATAR of 98, you pretty much have to be in the top 0.4% of the state and getting raw marks well over the 90s. Also, while 2 Unit does scale significantly better, it'll only drag you down less; you still have to hit the lower 90s in raw marks to hit your target (albeit that's the equivalent of top 8% of the state... but keep in mind that you would be competing against Extension 1 students).
University bridging courses are available, but the success rate is calamity. Unless if you are really willing to absolutely annihilate university maths bridging courses (which aren't that difficult, but you are given far less time to understand the content), don't bother. It's no substitute for two years worth of education.
So in summary, from my viewpoint, it would be 2 Unit, or no maths. Scaling-wise, do 2 Unit. Relevance-wise, do 2 Unit. However if you absolutely feel that you can't allocate the effort onto 2 Unit in the next two years (minimum 3 hours of work per week spent efficiently on maths) and that you won't be needing it (e.g. no finance, economics, actuarial, accounting, business analytics, information systems, marketing...), then no maths might be the better option. (I've just checked the B Commerce degrees of UNSW and USYD, and both has core units of quantitative analysis and statistics.)
Preliminaries mean very little (except as a means to enroll in certain HSC extension courses), so experimenting with 2 Unit maths, and your study habits/strategies in attaining efficiency in allocating effort might prove to be a resourceful way to spend your next year.