I think it's better to do various elements of music separately and combine them rather than try to do the whole song at once. It's a challenging problem. There is much less training data for example. I've worked on melodies so far: https://www.melodies.ai/, https://www.youtube.com/playlist?list=PLoCzMRqh5SkFPG0-RIAR8jYRaICWubUdx.

One problem: I only get at most 12 results per query.

You should have one more, I bought it. I was thinking to make something like this myself after Apple's search couldn't find a photo I was looking for.

I think doing the whole audio file at once is a worse direction than doing melody, vocals, chords, lyrics, beats as separate elements. I've focused on melody and I'm happy with my results: https://www.youtube.com/playlist?list=PLoCzMRqh5SkFPG0-RIAR8jYRaICWubUdx. I'm about to run a Mechanical Turk study to compare their quality to fully human hit melodies.

Getting there. I'm about to run a Mechanical Turk survey comparing 10 melodies (https://www.youtube.com/playlist?list=PLoCzMRqh5SkFPG0-RIAR8jYRaICWubUdx) I've created together with my AI assistant to hit melodies: https://osf.io/9nd6x.