I think I figured out the way to get the Koyanagi ending.
When you can take a detour, go to the park.
On 9/3 night, go to the park.
In the afternoon talk with Ryu until he invites you over.
On 9/4, in the morning, go to sartoli and meet Gon.
Once you meet gon, switch to Sartoli every single night until you fuck David.
And then another night when you watch him leave hot and bothered.
Starting on 9/8, go to Sartoli in the mornings until you meet Gon again.
Then go to sartoli at night.
After that, if you haven't made Koyanagi introduce himself in the park at night, do that now.
You need to make sure to choose "He wouldn't like that".
Now start going to the park in the mornings until David does the squats for you.
Then go to the residential area at night. He'll wash your back but don't let him cum.
Then go to the park in the morning and destroy him.
After that, the next 3 or so nights, you should get a memory with Koyanagi.
Just keep rolling with that until the end of the game.
If you don't see young Koyanagi, just keep going to kugai and picking all the options that favor Koyanagi.
And don't go to the island when the storm gets there.