disclaimer: im not going into econ and don’t do the subject and my school does fm differently in that we do both a level maths and fm at the end of yr 13 (just started yr13) but we learn all of a level maths and some fm in yr 12 anyway so i’m not in the exact same situation HOWEVER
PLEASE don’t assume a level maths in 1 year is going to be easy!! I CANNOT REPEAT THIS ENOUGH. i got a 9 in gcse maths and a 9 in further maths gcse too, but a level maths is still hard. i know many people who got the same grades as me struggle with even normal maths itself and many people dropped further this summer. the main reason why it’s so hard is not necessarily that the content is super difficult, it’s because of the pure speed you have to go at to cover all the content
in my fm class we covered a chapter of the textbook (pearson for both maths) in a week less at it’s most extreme. If you can be very disciplined and are genuinely ready to undertake fm, then please do it and don’t be discouraged, but also please don’t think that a 9 in gcse means it won’t be too bad.
as for the alternative subject, what i can tell you is some of the most smart/promising econ applicants from my school do the combo of maths, fm, econ and then chem/physics/geography
i think i’m general science is good for these kinds of technical degrees and they show you are very competent and they’re stem subjects. the human geography part is what helps w econ and there are many joint geo and econ degrees
i’d also like you to bare in mind even if you go with an econ degree, you can always switch to law later on (especially that branch of law that works with companies and things touching on econ)
hope i offered you some help!