The other maths have different boards because they're NEEDED. For methods, they're NOT.
Differences between 1/2 and 3/4 methods?
-Bulk of Probability (say, 3/4 of the AoS)
-Integration/some calculus things (say, 1/4 of the AoS)
So, the differences between 1/2 and 3/4 methods is one whole Area of Study.
Ignoring Foundation (because it has no 3/4 equivalent),
Further is very specific and extends on a lot of things from General - in fact, some things taught in General Mathematics are not covered in Further (because they're not needed) - two very different subjects, thereby requiring different boards.
And don't even get me started on GMA and specialist - the ONLY real similarities between the two are some conics, some vectors, some complex numbers. That's even less similarities between General and Further - equating up to about 4 Areas of Study. (if you want me to rant more about GMA and specialist, I will - just check any of my previous posts on the matter. I think most people are getting sick of me discussing it.
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Tl;dr - don't need separate boards, just chuck 'em together as they are - 1/2 people post in the 3/4 thread anyway, with very little care from anybody.