Love the project. Calibre is absolutely an amazing tool, but it never really adapted into the whole self-hosted era. I really like the focus on maintaining compatibility with the Calibre library. Personally the features I use the most are the folder watching and automatic book conversion. Now if there was support for multiple Calibre libraries...
Could you explain that in more detail? It seems to me that Calibre is totally self hosted.
I give an example. I use Calibre to import ebooks as epub (if they are not already epub, which they usually are,) maybe change their metadata and add a cover if they don't have one. The books go to Calibre's directory and then I sync to my phone using syncthing, like everything else I exchange between my laptop and my phone. I know that Calibre has a way to send books to devices but I think that it requires either a hardware reader or some interoperable app. I prefer working only with the file system. It always works no matter what I have on my phone.
All of that seems self hosted to me, or do you mean having a server somewhere at home or on the internet with the ebook collection?
I also use the editor to fix typos or to create ebooks from text files.