I use BookFusion. It’s a paid service but syncs the library and progress across platforms. Integrated with Calibre too, which I saw you mentioned you use in a different comment. Works great for me, but good luck finding something that works for you.
Ok that’s super interesting. That is much cleaner than my current way of downloading my books to my iPad, so I’ll have to check that out and see how easy it is to integrate
Oh,I think you’re right. They have their Calibre plug-in which is open source I think but I think in my head I conflated that with the whole app. My bad.