should i do AS further maths as a year 12 student?

(context) i did fairly well in my gcses by getting mostly 8's and 9's, but right now, im in year 12 and ive been doing furthermaths for most of the whole year (i did core pure, further stats, and decision) BUT ive been really **** at it.. for most of the year i did 4 subjects (maths, fm, physics and cs), and because i found fm really hard, i spent more time revising for it, but ultimately neglected the rest of my subjects. when we did the mocks, i got 2 A's in maths and cs, a C in physics and a D in furthermaths even tho i spent most of my time revising for it..
so now ive dropped the furthermaths and got my predicted grades (which i will use to apply for unis) to be A*A*B (for maths, cs, and physics respectively) thing is, I really want to go to Imperial for mechanical engineering (entry requirement is 3A* or A*A*A because i havnt fully decided which specific course to do), so now I'm considering doing an AS level in further maths to try and boost my chances (also i feel like i should mention that my form tutor and I have agreed to revise super hard and get an a* or a for my physics progress check in september to see if that will convince my physics teachers to bump my predicted to an A)
is it even worth doing the AS level in fm if i was always so **** at it? even if i got a good grade for it, would that even be helpful for me since it doesnt count as a full alevel and probably wont bring my minimum entry requirements down? am i reaching too high by applying to imperial because of its really high entry requirements?
im also trying to do lots of supercurricular activities and further reading and virtual work experience during the summer holiday to see if that would make my application stronger.. i know this was alot to read and i appreciate you taking the time of your day to listen to my struggles haha thanks