Should I play that prequel stuff before NitW or does it not add anything big?
I would definitely suggest playing them, but I don't think you need to play them beforehand. I played them both (on a plane, I think?) at around the 2/3 point of the game and that was a good time for it, but it's not like there's gonna be spoilers if you play them first so you might as well?
Longest Night is just kinda cute/amusing lore stuff with the main characters and is pretty quick. Lost Constellation is a deeper look at some specific lore and as a more traditional adventure game is slightly tedious despite being like an hour long, but its framework is that Mae's grandfather is telling her the story and they have a conversation as it progesses.
That conversation is great in itself, but it has a few callbacks at the end of the game that made scenes I loved even better, which is why I recommend playing them so strongly.