This seems like the group to ask. Is there a way to make Custom Column info more permanent? That is, to make it remain part of the metadata like title/author/tags when you have to restore or recreate your library? Just experienced this today and had to manually/batch re-add my Custom Column info.
And, to answer the thread question,

I mostly do fanfic and I use a Characters, Fandom, and Words (Count Pages plugin). For actual published work, I use the tags for any genre, descriptive info.