There are two sets of routes
-3 main(aka: long) routes, note: the alternative Ria route unlocks after you complete the 2 new routes, unlike the two new ones it doesn't take place after the prologue, it starts before it and replaces it with it's own continuity.
-7 bonus(aka: short) routes one for every girl, they unlock once you complete a single main route. Note: Ayaka's append route requires that you complete it once before you can select the alternate choice "defy destiny", it's not broken, the joke is in that the events of the first choice are meant to happen no matter what, the first time around because "they're destined to happen", but something that can be overcome, by just trying again, aka: just load a save or do the route all over again to get a different ending.(I think possibly a reference to the god legend from the first game, that gets a more in depth explanation in this game, but it might just be a simple joke)
I recommend doing Ayaka's main and append routes first since they're basically intros into what the main and append routes are going to be like, there's also a lot of meta fourth wall breaking humor/foreshadowing and comments that go something like: "hurray there's a new game, don't have overly high expectations it's a far less complex narrative that's mostly for fun instead of semi-in depth displays about the different pros/cons that come with different outlooks on life, it's just for fun, so have fun", if you save them till towards the end they may feel weird, her routes are comedy focused first and foremost.
Ah... disclaimer: There is still a little bit of messaging, expamle: in Ria's new main route there's a kind of sequel message to her original message about how "it's great to live life to fullest always aiming to be the best you that you can be so that you don't have regrets", this time it's about how "there's more beauty to life than just it's best moments, allowing yourself to appreciate and enjoy the little moments in life without minding that it might lead to minor regrets can be for the best, since avoiding burning out by trying too hard all at once can help you achieve better results in the long run", it's just not focused on in quite the same way as the original.
Tldr;
A good bit of extra/add on content, but not designed to be it's own original story, if you enjoyed the first game you might like this game in general but even if you don't care for the new main routes, you'll probably enjoy a lot of the append routes since they're basically just extra scenarios for all the main routes, the only exception being Ayaka's route it's a comedic "what if" route with no connection to it's main route, if you've never played the original game you should probably play it first otherwise you'll be missing a ton of context, since even the new routes take place "after" the events of the original game, relying on the original for characterization and world building.
