D1 is an oddball. The "maths" is primary school stuff and so the marking is incredibly strict and you lose a bucket full of marks for not writing down the exact steps expected.
If you like pure maths, especially calculus, you'll find much to like in FP3. Of course, if you don't like Pure maths, then why are you doing FM anyway??
To get an A in Further Maths, like all other A levels, you need 80% overall. That's 480 our of 600. TO get the A*, you need to get at least 480/600 and get an average of 90% in your best A2 level modules. That would be the best 3 of your FP2. FP3. S2 and M2.
If you're doing A level as well as Further, the exam board will shuffle your applied modules (D1, S1, M1, S2, M2) around to get you the best combination of grades. So if you do well in C3/C4 and I assume you did S1 as well, then your D1 could get swapped over to your A level and your S1 count in your Further. You don't have to do anything for this, they will do it automatically.