I am not sure what you mean by custom shelves approach.
If it is using a custom column then it is much the same except calibre won't add the column name automatically, but it is easy to just put the shelf name in the custom column as you add each directory using bulk edit.
I used tags because it is, I think, the default in the driver(s) and involved one less step.
Plus custom columns seem to mess up my Sonys
But it is a sound aproach and works well on Kobo.
I have several readers and on each one I use a different method of making my collections, and amazingly enough I don't find it confusing
But I keep all of my books in calibre because it is for me the most logical method of maintaining order
and I bought my first computer in 1977 so I could be considered kind of old school. But each to his own.
Slight caveat. Too many books in a shelf and/or too many shelves make opening the shelves very slow. One of these days I plan to put as many books as possible on a 32G SD card and see what happens, so I would be interested in how you come along as you add your collections.
Helen