idk too much about the scaling but i've never really seen ppl with math standard get over a 95 Atar. And also consult your teacher to see how they feel
i agree with everything else you said but this last part is lowkey not true bc its not that deep to band 6 maths std if youve at least passed your adv tests + 1 subject will not tank your atar by that much if you are good at your other subjects.
its always a good idea to drop subjects you are bad to subjects you are guaranteed to do better but i think the closer you get to a 99 atar goal you should consider keeping adv (not if you are flopping it and its unsaveable). 95+ is still doable tho
you are right tho, generally dropping from adv to std maths is most beneficial if you have struggled to cope with adv maths and not one singular bad test. i would acc argue a 55% on any adv maths test would be reason enough to drop to std bc it means youre strugglign hard enough that bringing that mark back up will need a lot of work that you will have to sacrifice from other subjects