I didn't really know where else to put this, but I just wanted to blab about my thoughts while playing through all the songs a bunch of times. I'm coming from this as not a hardcore rhythm game player, but i have played a lot of them(i'm like in the middle), I also have experience with gamedev and testing.
I agree with some of the other comments that the difficulty spike from the tutorial to first battle is stark, mostly cause the tutorial doesn't teach the difference between single and double lines, something i didn't even notice was conveyed visually + labeled till after playing through all of them lol. So constantly I was confused on when it was my turn, and sometimes I didn't even realize it was my turn and would miss a whole line. I don't think it's clear enough when things switch, I imagine this might be fixed with there being more easier songs to build up the skill of reacting to the current visuals.
I don't understand why the portrait covers the end of the music line, I would consistently miss a note that was placed there or think the line was over when there was still a beat happening in that section. The overlap makes my brain think the line is shorter than it actually is. It being at like 50% opacity doesn't help, I'd rather it be tacked onto the end- there you could make it opaque. The play area should always be clear imo, unless it's being done to add difficulty.
Apologies for the big wall of text, or if you already know all of this. Overall though this is really good, once I did understand what was happening I enjoyed my time a lot.