Also, main categories should be able to hold notebooks as well as subcategories. In all "normal" folder structures folders can contain files AND folders.
Top level view would then contain uncategorised notebooks and 1.level folders (= main categories). Some more sub levels would be nice.
Some kind of tree view or MacOS Finder-like view would help to keep track of location.

I had added an idea which is similar before I ran across this posting. I was also looking for a visual index (with folder icons in a grid) which would allow a quick tap selection. I think this should be the default view when opening the app. I'll vote for this posting in the hope that is will get some attention.

At its core, this is essentially the same idea as the one called "Main view with Master categories" among other requests. (I think so.) We can vote this up more effectively if we pile on to one feature request. Since you are reading this here, let's choose this one, yes?

A main view of categories (perhaps using folder icons of varying colors) would be visually user-friendly and would allow faster/easier choice when I sit down at a meeting and first open the app.