Audiobookshelf has very basic support for ebooks but it doesn’t compare to specialised software. Kavita has better management, and it comes with a built-in OPDS server so you can use any ebook app that supports that. I also don’t know if Audiobookshelf can actually be used for ebooks without having it as an audiobook as well, never tried it.
You didn’t “just need Nintendo Online”. You needed to have had it for consecutive two years as either the sole owner or head of the family account, 50 hours of having played games plus, crucially, having agreed to share that data with Nintendo, as well as having agreed to marketing emails, all prior to their announcement. Or you just preordered it literally anywhere else.
Oh, that is a new development I must have missed. Initially it had pretty tough restrictions, but they must have lifted them now. Thanks for letting me know!