Contents of Books app are not backed up anywhere. Need them backed up

I have hundreds of books in the books app. I had to install them one by one manually, because you cannot open multiple files in Books from the Files app this way - it only adds one book from the 1200 you select if you try this.

It takes a long, long time to put them back in the books app every time I restore the iPad from backup.

Books are not backed up to iCloud because they are not purchased from Apple. Local backups do not have them for some reason, either.

Apple Support doesn't even understand the problem and keeps telling me to buy iCloud storage, which DOES NOT EVEN BACK THE BOOKS UP.

How do I actually back up the entire contents of my iPad LOCALLY such that restoring it from backup doesn't delete all my books?