Copyrights for Series/Franchises

Hello everyone,

As you may know from my other posts, I’m currently working on a postcyberpunk space opera series. I plan on having somewhere between 5-9 books in the series, along with at least two stand-alone novels to fill in different gaps in the story (think Rogue One for Star Wars or A Ballad of Songbirds and Snakes for the Hunger Games) and different lore compendiums, bestiaries, and conlang dictionaries. I plan on copyrighting my series to prevent piracy before the books come out since I plan on doing little lore and character teasers to generate interest for release, but I need some advice.

For other multimedia franchises or book series based on earlier titles, such as the Marvel Cinematic Universe or the Dune continuation novels, I know they say something more or less like “Based on the characters created by X. X_____”, so I’m curious if I need to copyright the characters or the actual books themselves in order to secure something like this. Additionally, when would be the best time to copyright a series or individual book in a series? Do I even need to file for a copyright?

While I’m not worried about copyright infringement/piracy once the books are published, I do want to keep them safe before the release date because anyone can type anything into AI, and I don’t want them to put out a “fan-fiction” version of my work with the same title before it releases and cause a skewed perception of what my books are truly like.

Thanks for your help!