Had mostly dropped #Pixiv given it not having a good UX, but learned recently of a frontend for it, PixivFE, which also has #Atom compatibility! This should help a lot, with being able to go straight to the posts, there being an overlap of artists between Pixiv and other platforms, and Nitter being in a limbo while the author can't revert Twitter's C&D.

The most direct option is to go to a specific tab and copy the link from the feed icon, but I haven't dug around what changes between each feed variant (their links change slightly for Home, Illustrations, Bookmarks, etc.).

Removing the contents after the ? of a given feed also seems to work, though also not sure what changes for it either. But if willing to try this variant, and using the pixiv.perennialte.ch instance for the example, generically you pick an user's page, https://pixiv.net/users/some_numeric_ID, and convert to https://pixiv.perennialte.ch/users/some_numeric_ID/atom.xml.