@chocoxnot have you tried Latent Blue? the characters are a bit younger but they also grow as friends as they explore their love of diving together (i think they're supposed to become a couple but it's slow development like this).
i also liked Unlucky Mansion; the two main characters have kind of a similar dynamic (male lead is kind of the cuter needy one, female lead is the solid "rock" of the relationship) and it's more mature than Latent Blue (as in the characters are older so the themes are more adult).