we did fsmq last year at my high school, in which we had to self-teach it too. i think it is manageable if you're good at maths (if you're predicted a 9 you'll probably be able to do it
). i started teaching it to myself quite late, but as long as you make sure to cover the main big topics (differentiation, integration, logs, binomial expansion etc.), i think you'll be fine.
that being said, only 2 of us out of about 10 got an A, with many people getting Ds and stuff. so i think it really depends on your mathematical ability. try some past papers then see what you have to work on!
although you don't need it for a level maths, i thought it was really useful, as basically everything we learnt in the first term in maths was stuff that i'd already covered in fsmq. don't prioritise it over other subjects tho... it's one of the last exams, so if you're a quick learner, you can just do the revision for it the weekend before. obviously this depends on how easy you pick up topics (harder when u have to teach yourself!)
i'd recommend that massive fsmq textbook. i didnt use all of it, but it was useful using the practice questions in it.
overall, i say you may as well try it, as not much is lost even if you don't get an a. (just focus on ur gcses more
)