Great gamplay!
GBDK has some improved compression support in the most recent release. If you wanted to revisit this it might be easier to do now. Happy to answer questions as well.
https://gbdk-2020.github.io/gbdk-2020/docs/api/gbdecompress_8h.html
Also, here's how I compress and decompressed the music in Petris (which took some work to keep under 32k).
https://github.com/bbbbbr/Petris/blob/release_1.1_gbdk2020/src/MakefileCommon#L1...