I just looked at the top twenty English-language results for a few different tags, and I counted the number of works in each tag that are strictly nonfictional.
This might be a little inaccurate, since I've only read each work's metatext (or took a quick peek at the work if necessary). But I think it's mostly correct; it's pretty easy to tell at a glance whether something is fic.
Nonfiction is, as you might expect, a catch-all for a bunch of different types of work. A sizeable minority of them are memoirs or personal diaries.
Essays looks like the best, most focused tag (of the ones I examined) for anyone actively seeking fannish nonfiction. Now that I know this, I'm definitely going to use this tag for all of my own meta that's in essay format.
The Meta tag itself seems to contain a lot of fanfic that breaks the fourth wall, which is why it's surprisingly low on actual nonfictional content. Essays, Metafiction, and Fanwork Research & Reference Guides are all subtags of Metaâso if a work contains any of those three tags, it will also appear in search results for Meta, but the reverse isn't true.
Whenever I post meta to AO3, my first additional tag is always Nonfiction (and now I plan to tag works as Essays when applicable). Most people who use AO3 are looking for fanfic to read, so when I post something that isn't fic, I want to loudly advertise that fact.
I also tag the specific type of meta I'm writing. For me, that's usually Character Analysis. I've also tagged for Headcanons, Speculation, Analysis (of which Character Analysis is a subtag), Criticism of canon, andâbecause of my extremely niche hobby of assigning hero titles to non-Homestuck charactersâClasspects (Homestuck).
For character and relationship tags, I follow the same rule as I do with fanfic. Is this work about a character or ship, or does it just mention them in passing? I tag for the former, but not the latter.
Here are some (non-exhaustive) examples of meta works that I would probably add character and/or relationship tags to:
An analysis of two characters' canon interactions and relationship dynamic
A comparison of one character's canon vs. fanon characterization
A gushing essay about why your favorite rarepair is a really good ship that everyone should consider shipping
A list of headcanons about a specific character or specific ship
Basically, will fans of X character or shipâthe people who comb through the X tag looking for more content about Xâplausibly be interested in your work? If yes, I would tag for X.