One thought per note is one of those things that I strive for, but rarely achieve. When you create a note, you rarely know what specific thing you are writing about. That comes in time and as you re-analyze a topic you'll start to see where different facets come in. Also, the archive should reflect your mind: topics you care about will be detailed and have one thought per note. Topics you dont care about will be generic and more nebulous. PHD's have specific knowledge... why do we think we can achieve such specificity for every one of our notes?
I don't see the problem with achieving one thought to a note. It's up to you what constitutes a thought.
The big disadvantage of generic and nebulous comes with linking. Very many notes can link to a generic note, but only a very small % will be ones you want to follow whatever you're trying to do.
I don't see any difference between a PhD or a minor new difference either. The quality of the thought might vary. The specificity might vary. Knowledge should vary. But shouldn't be a problem with having one thought.
There could be a photo of a sheep in a field.
A very small child might think nice furry animal.
A slightly older child might think Shaun!
A butcher might think ready soon.
A farmer might think selenium deficiency? (mostly noticing the lush grass).
A thought is a single focus for what was in the mind. More foci simply means extra notes.
There's no reason to push yourself to have all the thoughts you might possibly have unless doing that is your purpose.
Revisiting the note or topic and adding new notes with new thoughts when there's a reason to is sufficient.