I ended up using this to download all the azw3 files for my kindle, it's probably not the best you could do, so feel free to use it as reference for yourself if you might do something similar.

  curl -s https://standardebooks.org/opds/all | grep -oE "/(.*).azw3" | sed -e"s/^/https:\/\/standardebooks.org/" | xargs -n 1 curl -O

P.S. Calibre seems to only work with them and the kindle when you send over USB (I keep my paperwhite in airplane mode, signed out, ads disabled)[0]

[0]https://news.ycombinator.com/item?id=20596300

For iOS there’s something called KyBook which reads that opds link and shows a list of books with cover art, book name, author and tags pertaining to the books subject matter. It does not allow one to search the opds library in anyway. I was able to download a book using the app, and from within the app move the book to iCloud. From within the Files app I found the KyBook folder, selected the book and using share opened it with iBooks and it is now in my iBooks collection. I found the app here https://www.maketecheasier.com/best-ebook-reader-ios/