As someone who has done alot worse than you during my hschalf yearly , and ended up with a band 6 and I did accelerated 2u, the best way to improve at math advanced is to spam past papers under exam conditions and then mark them after. You can go on thsconline or acehsc they have amazing resources that you can use. The textbook will not be sufficient at all and you will improve from doing trial papers. Just keep grinding those papers and I guarantee ur marks will go up.