Here's my feedback:
This was probably the first rhythm game I've played after like 10 years so I have no muscle memory or expertise when it comes to them pretty much.
I had a lot of difficulty with the game and I didn't realize there was an easy mode for the songs until I beat the story mode (although I'm not sure if I could've changed the mode in the story anyways). There's a lot of things that aren't really well explained, like how expression works or the double tap (?) input that's needed for the final part in Cream Cheese Icing. As someone else pointed out the character icons kind of get in the way of the beat instructions when they pass over them.
I also had some issues with the double line timing, as in not having a visual indicator as to when the top line ends and when the bottom one starts Without looking at the end of the top line directly as that wouldn't let me see the inputs needed inmediately as the bottom line starts.
After I set the game to easy mode I couldn't notice any major differences in difficulty, at least not with the only song I tried it on (CCI).
There were some parts when I would make a correct input to the beat and even though it would play the beat sound, Melodi wouldn't sing and it was left to chance if it registered as a correct note (especially in the first CCI double line part).
CCI felt really difficult considering it's right after the tutorial, and all the other songs felt around the same difficulty.
I didn't have any problems with the overworld, although it kind of felt empty being so big but having only buildings for important areas with a couple of exceptions.
I liked the songs themselves, none of them sounded off (without taking into account me being horrible at rhythm games thus ruining Melodi occasionally).
Animations were really charming too, although it's a shame I couldn't look at them in full since I had to be constantly looking at the notes.
Take all of the gameplay criticisms with a grain of salt since as I've said I haven't played rhythm games in a while and as such, it's possible all of them are skill issues on my part. I hope the rest of development goes well for you.