Here is the full list of all 15 Sakura Nova achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Arisa ending
Every ending achieved
Kaguya ending
You saw one of Kaguya's gorgeous appearances in a dress!
You finished the game super fast!
Both girls ending
Special scene with Reika!
You defeated the enemy!
You defeated the enemies!
You played the game in both languages
You played the English version of the game
Are you ready to experience Sakura Nova?
You played the Japanese version of the game