Started ok but is becoming boring and weak
I think it'd work better if the MC wasn't so formulaic in his spinelessness.
The rest of this is entirely my own take and perspective - but the mistaken love letter switch that's also been left not-clocked by
any of the four is a sweet setup, but for someone that a girl like Nanase is ostensibly supposed to fall for, the MC is just....the most stereotypical wishy washy male character possible for a romcom, right down to the "tragic backstory social fear of women" personality.
The premise is actually halfway novel, but the characters that have been inserted into the scenario are as cookie-cutter as it gets with nothing resembling originality about them. And for a story that really needs to build off the strength of the characters and their interactions and relationships, they're all a little too predictable, which kinda wastes the concept, a bit.