So I decided to put this in between some of the main games routes, because it felt right. The start of Sana's route is shared with Miu's although only the very start of it. For anyone who doesn't really care much for the other girls (based on design for example) you can totally read through this without touching the main game. When you start it up, you will be put at the very start of the story and will have to go until the opening movie. After that you get an option to skip the common route and go straight to where it starts diverging from the main game. I do recommend playing Miu's route before this, because I personally feel like the impact is just different. As you may imagine, you get the most Sana screen time in her route.
Quality-wise I didn't find too many things that stuck out. There was some weird translations here and there and some typos, but it wasn't to an extend that you actively start noticing it too much, not like the start of Ange's route in the main game.
Other than that you can expect everything you get from the main game. Amazing art, great music and good story telling. Reading the entire thing took me around 12h as you can see, but depending on how fast you read, you can probably finish it in less. Because Sana gets an entire standalone dedicacted to her, the story is much more in depth than any of the other heroines. If I had to voice any complaints, it is that the story at some points feels like it really wants to delay development, although honestly considering this is supposed to be pure first love, that is fairly accurate I feel.
The price might seem very steep, but the production value of this VN is also just really high. I do recommend getting the Bundle either way. I imagine that if you play this standalone from start to finish without any skips, you might still get around like 20h out of it.
To give this an actual rating, I would say it is around 8/10, simply because there was some plot points I did not super enjoy, but this is really just a minor problem to me anyway. Sana is unbelievably cute and her story, to me anyway, is really touching. She grows a lot through the course of this VN.
As an afterword: I played this with the R18 patch, so I dont know if you are missing any major parts if you don't have it, but even if you are not interested in that content, i would recommend getting it anyway. You can download it for free, since I am not sure about steam's link policy though, I recommend just checking on VNDB. You can find a link to it there.