Nice work! I liked the spring mechanic and how you had to switch back to human form to move faster. Would have liked to see a similar thing with the slime. Would be a bit harder to program, but what if the slime slides down hills and you have to use the momentum from that to jump faster in human form? (As an example)
That first jump on the last level was pretty (overly?) precise, took me a few tries to make it.
Music and sfx are always good for a game, and you can hook them up fairly easily in godot/unity. You can get some free CC0 ones at opengameart.org.