I'm not sure what AQA is like but its mostly the same between boards I think. D1 is horrible I think, it doesn't fit in the same way as other maths modules. It doesn't require as much calculations and thinking through, just memory of a pattern to do things in. So its easy to lose marks on it because they are strict about small things. I'd retake it if you think you can push it up, but its a pain.
I wouldn't worry as much about FP1, you were close to an A and on Edexcel it was quite different to FP2 and FP3 so it'd be difficult to do them all at once. You should be able to carry on doing further maths with a B in FP1, otherwise they wouldn't have that grade available.
As for M1, this depends on you. Personally I hated D1 and would happily do M1-M3 again in place of doing any more of that. Its pretty interesting too, although M1 is at such a basic level that its hard to relate it to real world ideas (which is one of the reasons I liked it). I definitely found it easier to get high marks in the mechanics modules rather than D1, because the solutions felt more concrete in a way.
Also remember that all your applied modules will be moved around so you get the best grade in both maths and further maths. Do you need the A in further maths for part of an offer? Just wondering because I saw a lot of AAB offers with further maths, but sometimes I was told the further maths could be the B, and maths and another subject as the two A's.