Sorry, just seen this, I would say it’s quite manageable, just like any other A Level, but depends if you’re doing it part of a 4 or 3 A Level course. Personally, I believe maths and further maths feel more like one BIG A Level rather than 2 A Levels (even though you’ll have separate exams.)
By the time that you finish further maths you’ll probably need little revision on A Level maths because of the extensive nature of further maths. The content is very accessible with practise, start to think about about which modules you’d like to do, personally I just stuck to what my school offered (further stats and mechs) and the mechanics side of it I found very difficult considering I dont also do physics, so that’s something to consider as well.
Graph theory is not taught in core pure, however I believe it may be taught in further pure, but im not entirely sure.
Hope this helps.