I don't mind that there are other size options out there. I loved M&S socks, they were really good quality. In a way I do have the same problem you have, 3-5 is just too small for someone with a shoesize 5 but 6-8 is so massive, I get blisters because there is too much fabric around the heel.
Still, the average female is a size 5-6. I have friends with size 7 or 8 and I do know they struggle, both moved to mens socks. In an ideal world you would get adult socks in smaller size ranges and overlapping but I assume the production costs are too high.