I have been playing through this again and thought I would add my feedback. Didn't hear back from you but I'll just tell you what I think.
- Level design in quite nice, no frustrating learning curve, easy to understand what must be done, no particularly frustrating jumps to be found - so it ends up being quite fun to move around through the levels.
- Since you have chosen to use the dark tones, this will work well on emulator but on the DMG, it is super hard to see what's happening when the screen scrolls. Some platforms are hard to see because there isn't high contrast against the dark black background. If you were to implement some white highlights on the top of the one way platforms (like your boxes), that would help readability dramatically. I find its always best to not use a black background if you want to design for the DMG. Having said that, I have seen some GB games use the "night time" scene really well (Ninja Gaiden Shadow is the best example) but even then, they stylistically imply its night time rather than use the black tone everywhere. GBC development is best for more moody stuff in this regard because it doesn't affect readability on the GBC screen so much.
- Inversely, there are some chains with a white highlight that are background elements, they are more prominent visually than the platforms so there is also readability issues with that. I recently wrote a 2 part article on readability if you want to check it out (https://gbstudiocentral.com/tips/dwf-chapter-6-readability-part-1/)
- Finally, the hit boxes are quite frustrating, particularly the side-ways facing spikes. I assume this game was developed in GBS2b5 so therefore, you can't change the hit boxes. If its migrated to GBS3.0 then you would have much more control - but that is no easy task! I have tried that myself with my own projects and it can be quite the task. If you are not doing that then the jumping hit box could be fixed slightly by moving jelly buns jumping animation over a pixel or two in the PNG - I have been hit so many times with the side spikes when its clear that jelly bun never touched the spike - so its really hard to make that second side spike jump (after the waterfall and body of water) I think the player drop off would be highest here. I stopped playing here the first time I played due to frustration.
- Otherwise, I reckon all the nice little touches in the game are just wonderful. You definitely have a great eye for polishing the enjoyment factor - with the sparkly effects, bubbles, "chomp", stuff like that. I wouldn't expect you to update this game as you are likely working on some other no doubt really nice game at this point. But I wanted to share my thoughts with you so you have some useful feedback to take into your future projects. Good luck with GB Studio development and great work!