No, I had assumed that wasn't what was wanted:
But I do think how "easy" it is for composers is odd compared to how "hard" it is for lyricists/performers. A ratio system may be too subjective and hard to quantify... I don't think an objective system that achieves what is wanted is impossible though.
What about instead of one credit, someone needs more? For example:
If someone has 3 different albums (in any role) they are counted for Category 3. This may be problematic if one song ends up on many albums, so how about 3 songs/tracks and 3 albums?
Maybe just any number of albums for 3 unrelated products?

Something along those lines should avoid including artists with only a very fleeting connection to Category 1 (e.g. 1 song) but still not exclude more-relevant people. Even just "5 tracks, anywhere" would be enough for that. If the general policy is specific enough to handle most possibilities well, the staff discretion would be left for the remaining small number of anomalies, such as the people already identified in this thread.