I was basically in your position too. I was even asked if I wanted to continue the accelerated course at my school in Year 9 because my marks were that low. It didn't improve either in year 10. Some preliminary exams I scored around 60-70% (like you). However, I went through the whole way, and now I am studying only 8 units for my HSC in 2022. I think you should embrace the opportunity of accelerating, as it will allocate precious time for your other subjects.
I didn't truly begin performing till Year 11 term 1, where I ranked 8th (first time in top 10), but thats because I worked a ton during the holidays, and since I had no HSC pressures aside from maths, thats pretty much all I did. Look, I initially came from receiving 60-70% in exams pretty frequently, to receiving a 48/50 for E1. Everyone is different, so if everything is going over your head and you think it is impossible, then you consider your options, but you said that your issue for low marks was because of a lack of practice and study. In that case, you definitely can improve as it doesn't seem to be a conceptual issue.
Worst case scenario, you mess up in the HSC. In that case, you have the privilege of redoing maths with 2x the experience of the non-accelerated cohort. In retrospect, I still have regrets about some results and study habits, but I am way, way, way better off now that 1/3 of my week is a study and I can focus on my other subjects.