This is one of the best Extra stage jam games I've played. It's just amazing. The art is cute, the story is easy to understand (very important when you just have a few lines of dialog to work with), the subject matter will be relevant to most players / devs (idols, popularity).
Of course, the most important part is the gameplay, and the game excels here as well. It seems as if a lot of thought has gone into the patterns: The non-spells often act to establish the mechanics / flavor of the upcoming spell cards. There is good variety in how you dodge things: macro/micro, even high up on the screen sometimes. The difficulty is just right for a jam game for me: it took me around 10 attempts (not counting instant resets where I crash into a bullet on the first spell or something), so it always felt like I was making progress and getting better at dodging the attacks. The game nails the Extra stage flavor where a lot of the attacks aren't that hard to dodge once you've learned the gimmick.
Not only are the attacks well-designed from a dodging point of view, they're also on-point when it comes to the theming. Even without reading the dialog, you can tell what's up when the glowsticks come out, the hearts go in, and the way the survival spell winded down is just excellent storytelling through gameplay.
A few small nitpicks:
Firstly, the score really should reset upon death. This feels like a programming oversight rather than a design choice. It's not a big deal since the scoring system is kind of ill-suited for a boss fight anyway, but anyway. (I guess that's my biggest nitpick.)
Secondly, the way the glowsticks go left before firing their bullets is a bit rude, since it makes it hard to line-up visually before the lanes are out. I guess this might be a reference to how people actually dance with glowsticks(?), but gameplay-wise it feels a bit unintuitive.
Lastly, the two final parts of the final attack could probably be a bit harder. They are far easier than the first two phases, at least for me who hasn't had to opportunity to sit down and practice these spells.
The only reason I'm being nit-picky above is because this is really close to being a 10/10 jam game. I could see this getting #1 or #2 place in pretty much every category. And it's not danmakufu? What did you use to create it? It must have been a lot of work to get the Touhou feeling right.