I really love the game. I've been craving a new rhythm game with good music for a long, long time now.
I came from the Guitar Hero scene from 2006-2014 and used to be in the world-level competitive scene. Having years of rhythm game experience and love for the genre. So now that you know where my feedback is coming from.
I very much love the overall concept of the entire game. Absolutely adore it. The newest level/track are amazing. The delay functionality being in the core development process is also critically important and probably the best decision in the entire development process that you could have possibly taken, so I'm very happy to say that you're blowing it out of the water.
Now, I've been noticing people saying we need better charts, I do agree to an extent. I felt like Medium on the new track was pretty solid, not too hard, not too easy. A perfect middle ground where it's just hard enough you need to focus but not hard enough that you're clenching trying to hit everything. Though, I also know charting songs isn't the easiest thing in the world to do. It's hard to find the right balance because what may feel easy to one person is going to be difficult to another. And as a developer, it's even more difficult to chart because you have your personal experience to go on and then you're trying to adjust it to other people. It's not easy.
Now let's talk about Hard on the second track. To be blunt; it's a cluster %?!#. The issue is the notes are all white. There's no grading to them whatsoever. So when the notes are placed very close together in long rows such as the triple notes bouncing around the screen with single notes between, everything starts to bleed together and your brain just says "press them all" which, is regrettably something the game does not punish you for doing other than getting a "yeah" instead of an "amazing." And while this isn't a bad thing, it's also not a good thing. I don't think we need a rainbow of colors for it, but potentially just grade every other note to be a little bit darker than the one before it, just so there's something to differentiate the notes so you can clearly see what's happening. I would say put some vertical lines to section off each note but that would further distract from the background, so I don't really think that's a good idea.
My advice for charting difficulty would be; Easy = 2 fingers. Medium = 3 fingers. Hard = 4 fingers. The concept is they should be doable with those fingers alone. So someone who can play Hard well can play Easy well with two fingers for example.
I hope that anything I've said was of any help to the future development of this game! I will definitely be following its progress.