This started off so lighthearted and innocent and then it went hard and I was not ready for it but damn it was great. Incredibly well handled writing-wise - I went into the story blind by seeing someone stream it first (and not thinking to check the game description beforehand), and it was great to notice things being hinted at and danced around early on, then have all of that foreshadowing actually become important later. And when things hit the fan in the middle, the way the tension builds really makes Sekibanki's fear feel palpable, and her internal struggle completely understandable. (Not that she wasn't relatable before, I'd want to bonk my friends too if they pulled the stunts Kage/Waka did.) Also Kogasa being heroic in the face of the closest thing Gensokyo has to a Terminator was as unexpected as it was cool - I guess she's good at surprising people after all.
In terms of technical/game stuff: animations and music choice and CGs/CG placement were all good and did an excellent job of enhancing the story further. Plus the optional Q&A at the end was nice.