This looks very similar to Trilium Notes [0] which I use.
Zim's UI looks more dated and simpler, but underneath it's extremely powerful.
The UI is utilitarian, simple and efficient. It stores all of its note files as .txt so there's lots of other ways to access the info (e.g. grep).
Zim is extensible with a variety of useful plugins too.
Its simple file structure also allows me to pair it with syncthing [0], and sync all of my computers with each other seamlessly. So I have my own self-syncing document trove and knowledge base without any considerable effort.
Using simple apps and integrating them is much more powerful than it looks, and it's one of the simpler things that I enjoy immensely.
I'm somehow not very comfortable putting my ideas or research into some graph db I don't own.