Obsidian: https://obsidian.md/

  - All content stored as flat files on your hard drive.
  - Organised by correctly named folders.
  - Can embed images. Will link or autocopy them to the appropriate folders.
  - Is great if you like markdown.
  - Dark theme.

  - Infinite tab indentation synced in google docs, in google drive, since I already pay for google anyway.
  - todo.txt

Login to recommend Obsidian, too. After several months of usage, I even created a regex plugin to enable me to build knowledge database even faster : https://github.com/No3371/obsidian-regex-pipeline

- See/Hear some Japanese words I don't understand

- Search it on goo (Japanese dictionary website)

- Clip it with Markdownload (https://github.com/deathau/markdownload)

- Format it with my plugin

- Read the content, weave the backlinks

Keep doing this then I have the super complicated graph in the plugin demo gif you can see.