After examining the game's script in RPGmaker, I found out that there are two hidden requirements to achieve this ending. You might already know about the first requirement from other sources, but I personally never found correct information for the second requirement anywhere else (it has nothing to do with fishing), so hopefully this helps out anyone who got stuck.

The first requirement is to earn 5 or more affection points with Miyako Sakuma during Day 1 in Azakawa Village. You need to pick the right dialogue choices when you meet with Miyako and Officer Mochizuki multiple times when you explore the village.

The game plays a distinct sound effect when you choose correctly. Here are the screenshots with the right answers:

During the prison section, you need to talk to the non-hostile ghost in the room with the diggable mound and give up a luminous stone to them. Make sure you have enough stones left over, otherwise you need to reload to an earlier save where you didn't take too much damage.

After Miyako is trapped by the snake and Shiori heads back to the fork in the path, there should be a decision to return instead. This only appears if the requirements were met correctly.

After selecting the second choice, the 6th ending will play, congratulations!