If your game is 32kb or less, go on Catskull Electronics' 32kb cart and select Custom ROM. It's really cheap and works effectively.
Viewing post in Baunsudaun [Game Boy] comments
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...