Your suspicion is correct and it is worth stating plainly: for retrieval, search beats structure, and it has for years.
If you can remember any distinctive word in a note, search finds it faster than any hierarchy. Filing exists partly as a habit from paper, where search was impossible.
Which means the four reorganisations changed nothing because they were solving a problem you did not have.
What structure is actually for, and it is a much smaller job than people assume:
Finding things you cannot describe. Search needs a word. Structure helps when you are looking for the thing about that project, whose contents you have forgotten.
Seeing what you have. A list of projects tells you what is active. A flat pile of four thousand notes does not.
Separating states. Draft against finished, active against archived. This is genuinely useful and it is about status, not subject.
So the useful minimum: a small number of folders reflecting what you are doing, not what things are about, and search for everything else.
The reason subject folders never work is that a note is usually about three things and you have to pick one. Status has one answer, which is why it is the axis that survives.