Calibre has a feature that will fetch a rss feed or (IIRC) scrape a site and make an epub from it.

Nope! Every source I care about either has an RSS feed, or I've been able to make a custom recipe [1] to handle things like auth for paid sites and whatnot.

[1] https://manual.calibre-ebook.com/glossary.html#term-recipe

epub is ZIP with HTML+CSS+images+some metadata.