Is it worth sitting CIE exams in year 13?

Hi all, thank you for taking the time to read this and I hope this finds you in good health. My question is whether it is worth self studying for CIE exams in my last of year of highschool in NZ if I am aiming for a top university. I would like to know how it would affect my overall academic evaluation. This is because my school only does NCEA, and due to certain circumstances I have not achieved at the level I believe I am capable of.

I did Cambridge math in year 11 and found it much more interesting and stimulating compared to the NCEA math curriculum and as a result will probably sit a couple A levels next year regardless. However it is always nice to know how much weight one’s formal education carries.

P. S. I would also appreciate if certain subjects of the A level curriculum could be recommended to me as physics and chemistry (my two most proficient subjects after math) are unavailable at the local examination center. I was thinking History, Economics and perhaps Classics.

P. P. S. What is an above average number of A levels to take.

Thank you sincerely for your consideration,