Here's the walkthrough I've seen posted in the Kagura discord. The VN is actually very simple and unsubtle as fuck:
There are 7 endings and two main routes. Which main route you get on is determined by which girl you choose 5 times during the day (night choices don't count at all) before the route split. You'll know that a route split happened successfully from the fact that a video will play after the fireworks thing and the game will continue.
1) Mayu GOOD: Choose Mayu at least 5 times during the common route, then keep choosing her after the route split.
2) Mayu NTR: Choose Mayu at least 5 times during the common route, then choose Airi three times after the route split, then don't choose to cheat on Mayu with Airi.
3) Mayu BAD: Choose Mayu at least 5 times during the common route, then choose Airi three times after the route split, then choose to cheat on Mayu with Airi.
4) Airi GOOD: Choose Airi at least 5 times during the common route, then keep choosing her after the route split.
5) Airi NTR: Choose Airi at least 5 times during the common route, then choose Mayu three times after the route split, then don't choose to cheat on Airi with Mayu.
6) Airi BAD: Choose Airi at least 5 times during the common route, then choose Mayu three times after the route split, then choose to cheat on Airi with Mayu.
7) Ryuichi BAD: Don't choose either girl at least 5 times during the common route.
Note, the bad-endings where the MC chooses to cheat with the girl he's not a route with are a single sex scene and then some minor story/dialogue and the game abruptly ends. It's VERY lazy... Those scenes even reuse CGs from other scenes and don't unlock the CG in question in the gallery. Not sure if the scenes themselves are even registered.
I haven't finished playing through all of this mess yet. Mainly because I also have other things to do... but I can already say that the writing is pretty damn bad. Much like the game before this one, Avantgarde wanted to have their cake and eat it too, and they royally failed at it... but I'll leave a full series of thoughts later, once I finish reading it all. The girls and the sex scenes are hot at least...
EDIT:
One additional note. The exclamation marks during the day (pre-route split), unlike in other VNs/games out there, do NOT mean important events in any way. Before the route split, during the day, they basically mark events where the MC goes out on his own, rather than with anyone else. You do NOT want to pick these up unless you're going for total dialog completion and/or ending 7. Some exclamation marks may pop up at night for some special mini-events, but night choices don't matter anyway.