The main mistake here is you're thinking about "story" in traditional form of plot growth. This game is more character/MC focused instead so the story will seem like it goes nowhere to you, but in reality, the story is the MC which (unfortunately to you) includes meeting a bunch of characters as his social circle expands.
The reason why I'm ok with this is because these characters are introduced logically (ex: meeting basketball people because he's trying for the basketball team, meeting mom's boyfriend family, etc) while still involving the established characters. Not all of them at once, but for example the book club introduced a bunch of new MILF characters but also allowed character relationship growth with Sasha. So it's not like these characters are really "nowhere to be seen" as you claimed. And because there are so many established characters, only Bella and Nami would appear regularly (and most likely Mila too once they've lived together) while the others will have to take turns. It's just the nature of the game when there are so many characters to begin with.