papers from 2002/3-2006 are far too easy, I didn't bother at all.
It is not necessary to do the exam papers more than once. A good 6-8 past papers should suffice enough for 100 UMS standard.
I do not recommend solomon/delphis papers. I just find them inaccurate and they didn't really do anything to help me.
Exam papers are good but you just need to understand why what you're doing is the correct and most EFFICIENT method and why it works in that context (to a certain extent). Questions are always meant to be solvable, however learning how to do it properly rather than just being lucky because you've seen it before is a much more efficient way of sitting exams.
A level maths is basically just proficiency, if you can consistently score 75/75 on a module, leave it for a while (no more than a month) and go onto a different one. I sat C1 C2 FP1 S1 M1 M2 and I can say that once you get onto applied modules, more so mechanics, exam learning is a bad way to tackle an exam. 2016 M2 was a tough paper and would have caught many people out that didn't understand what they were doing.
It all depends on your ability and how fast you pick up the content.