As a Taiko player and someone who considers themselves a fast typist, here are my gripes with the current state of the game:
Chords are hard to read - This is a big one that I have no idea how you would fix. Harder maps are near impossible to play because having three rows and then having chords that span all three of those rows make reading them insanely difficult, combined with the fact that you already have a very tough challenge on the middle lane. A solution could be only having two rows by merging the numbers with the letters or the letters with the space (which would bring about the problem of it being mistaken for a 'u' or a held note), but as your charts aren't designed with this in mind I'm not sure how plausible that would be.
Arbitrary coloring - This further complicates reading. Having the same key be a different color is very off-putting, especially when you have fast sequences like 'kkjjv'. You could have the keys be a fixed color based either on which hand is used, which finger or which row the key is on. Ideally, these would all be possible choices from the settings menu.
Unforgiving hold notes - More often than not I found myself releasing the hold notes right as the hold line ends, causing me to miss. You could maybe allow some leniency for the release or give out less points if it was let go of a little earlier.
As for the UI, I think it's pretty good. It would be nice if the arrow keys behaved the same everywhere - as they do when you're picking a song (Being able to hit them again in the middle of the animation) to get rid of some annoyances. Giving the Escape key the functionality of Backspace here wouldn't hurt either. You could also consider using the Escape key to resume the game, but that's up to you.
As for the technical side - On Linux, after finishing a song and hitting a key to display it in the graph, you can't get rid of the flashing animation. (Maybe this is intended?)
That's the only thing I noticed, apart from the other problems which every game that has a build for Linux seems to have.
Overall, I really like what you've done and wish you the best of luck with this game. <3