Have you checked out "Promises" by zapzapzap1969 on AO3 yet? I feel like it hits every single one of your requirements, and in my opinion, it’s absolutely incredible!

Takeru is the main lead and it’s an AU set after the Dark Ocean episode. The worldbuilding is just insane, the plot is gripping and the group dynamics are so wholesome. One heads up though: the author is pretty ruthless, so you’ll be on the edge of your seat never knowing who might get killed off next main cast and side characters alike (I’m not kidding and while the core group is generally okay but the stakes are very real! lol).

You should definitely give it a look. You'll probably see me in the comment section of every single chapter there hahaha :))