IIRC, I just connected via SSH and replaced the files in the default screensaver folder. Can’t check anymore though, because I’ve sinced switched to a Kobo.

I don’t think there are many tangible benefits if all you want to do is read. You’re likely using Calibre anyway for sideloading books and it converts everything to a compatible format automatically.

The only somewhat useful things the jailbreak allowed me to do is add custom fonts, custom screensavers, install KOReader as an alternative ebook viewer (although I rarely used it) and prevent updates because the interface became a lot worse after an update at some point.