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you can export epubs from https://fichub.net/

I use a calibre add-in https://www.mobileread.com/forums/showthread.php?t=259221

It sort of works ie some stories just work others just get the first page.

Fanfiction.net is trivial... apart from it having Cloudflare bot blocking turned up to aggressive levels. I've not seen an approach that works, other than using headless browsers to fetch the content.

headless browsers won't work by default for cloudflare captchas.

open source stealth plugins don't really work now either.

you have to use real browser fingerprints.

You can import a csv of all the chapter links, looks like it's just incremental numbering in the url

The issue is most likely cloudflare blocking most the best scraping methods. If the site can be pulled down with eg. wget or curl without a bunch of options that you definitely aren't writing by hand, pandoc can just be used to directly make an epub.