On a mission to improve my piano sight-reading, I've been systematically working my way through all major and minor keys, 'living' in a given key for a few weeks before moving to the next.
Whilst living in each key, I practice its scales, arpeggios, chords, exercises, compose something in that key, reading randomly generated music in that key, and dripfeed myself actual pieces in that key from repertoire packs I make for myself in that key (organized in graded order of difficulty from easiest to hardest). The latter activity (sight reading actual pieces in the key) is for me the most enjoyable (playing actual music is what I like doing most), so I kind of use it as a motivational self-reward for the trudge through the more dry work. Lots of scales, exercises, arps, chords, theory, then I treat myself to a sight read of a piece of music, and so on, with the pieces gradually increasing in complexity.
This system has been working well and I've improved measurably - to the point where sometimes the feeling of improvement can be quite exciting.
But the problem I've encountered is that for some keys there just isn't the material in existence to make a repertoire pack.
For example, I'm currently 'living' in B Flat Minor. Whereas for other keys I've been able to make packs spanning grade 1 through to 6 with plenty of material in, in B Flat Minor there's no material at all below grade 6 (and even at grade 6 and grade 7 there are just a handful). So when I'm wanting to ease myself in by sight reading lots of grade 1, then 2, then 3, then 4 material, it's just not possible in B Flat Minor due to very little composed in the key - and what there is being too complicated for my sight reading ability. So for practicing sight reading B flat major for actual pieces of music, I'm having to plunge straight into sight reading grade 6 and 7 material that is just painful for me to attempt and I'm not sure it's doing me any good.
So what do people do to become proficient at sight-reading actual music in keys where there is no material in those keys suitable for their sight-reading level? Do only the other work in those keys and just hope that getting better in general, in other keys, will translate (magically!*) into improvement in the 'less practiced' keys? Make up for it by doing more of the non-music reading work in those keys (and take the demotivating experience of not having actual music to read on the chin?). Something else?
Any guidance appreciated. Thanks.
*I say magically here because in my experience, being proficient in sight reading in several keys has not so far translated into being proficient in other unpracticed keys (though it does seem to help 'a little') - which is why I'm doing this whole 'live in a key' approach in each and every key.