I'm actually working on an all in one solution that sort of fills the roles of Readarr AND Calibre - without all the baggage of the latter.

Oh, and of course a KOReader library plugin that foregoes OPDS and works like the Kindle interface ("available books" synced from this backend, one tap downloads them, plus potentially adding series handling too in the future).

But right now it's mostly manual downloads + CWA ingest + OPDS via KOReader.