I love this review! Of course, I do not agree with everything, I feel the vocaloids used add a bit of charm + go with the feel of the game. It sets it apart from other games as it isn't all too common.
Hopefully, more people vocals from people are thrown into the mix for future characters. I think both would be neat.
As for the butter, it may seem jarring and forced a bit but I think it's good. The game is very silly and animated like a 2000s CN Show with certain expressions and moments going on. I believe the cuteness and silly vibe compliment well for this, even though, it's an unlikely duo.
The dancing butter won't be everyone's cup of tea but It's tame enough to enjoy and not be so off putting.
Lastly, I completely agree with the free miss portion. The note is just thrown at you and its difficult to prepare for it since it's right there.
I think the first note should come in like it's on a checkout conveyor belt similar to Taiko No Tatsujin and then after it hits the first note it stops and plays as normal. Sorta like a heads up!