I bought the game on the Switch and just finished my first playthrough. I started a second playthrough after credits rolled, and it seems like everything is going the same way regardless of the different choices I make. Are there any actual alternate endings? Or does the story play the same every time and you simply spend more time with different characters in between to unlock pics/poems?

There is an alternate ending (two, actually, though one is more of a nonstandard game over), but it's not something that you would think to do naturally.

HINT: The game remembers what you do even if you save and reload, up until the point where you reset it in the end.
OTHER HINT: Monika went crazy because, as president of the literature club, she knew the true reality of her world. But if something were to happen to her, the vice president would assume the role of president...

Without getting too into detail you should know there is a hidden master save that keeps track of all the scenes you've unlocked "during that playthrough" with the different characters. During the credits you might have seen some images scroll past that were grayed out, which indicated scenes you'd not seen yet.

I'll leave it at that, but wouldn't be bringing it up if there wasn't more to it than that.