It just so happens that I have a copy of TheBrain 8 but it has basically been untouched for the most part. I'll give it a go as well.

Stephen Zeoli wrote:

This may not seem "smaller and less complicated," but TheBrain would
handle this very well. You can drag documents into the Plex under the
"thoughts" (read folders) they belong to. You can attach any document to
multiple thoughts, if that helps (i.e. you're not restricted to one
folder). You can classify each document with multiple tags. You can use
the notes tool to annotate the documents. When you click on the
documents, they open in their native application.

This short tutorial demonstrates many of these functions:

https://youtu.be/FwZmesy5toQ

Of course, TheBrain may not work well if others need to access the files
from other computers.

Steve Z.