i think to an extent, you have a good idea, but aren't necessarily going the right way about it.
you are relying on the grades of pieces to exercise your technique in a pre-assumed manner. for instance if you go about this its like you're saying, i must learn exactly three pieces from each grade, because... just because i assume it is good.
it has good intentions, but really you shouldn't look at trying to create some formula which is going to make you learn the piano. what i would suggest for you to become more familiar with other composers and music, is listen to a lot of it. then learn a piece because you like it, providing its within your ability. its good to learn pieces in a variety of difficulties. for instance some pieces which are low down in 'grades', others which are more challenging for yourself.
but whatever the piece, you must consider how you go about learning it. every piece you learn, you must go about mastering every technical part of it that you cannot play right away (unless its note reading which is causing the problem, but then that should be worked on also), and whatever the difficulty of a piece, theres technical aspects you either can, or can't play. whether the piece be grade 1 or grade 6, mastering a technical aspect in it is equally useful to your toolbox of abilities. make sure you learn all your pieces perfectly by taking short sections that give you trouble, never be satisfied with fumbling through a few sections just cuz they don't seem to matter too much. we should not be fooled into training our entire technical abilities with technical exercises. the technical demands of the piano are enormous, but technical exercises can only offer a few. the only way to progress is by playing, and perfecting lots of pieces.
cadenz.