The Sage looked nice but the price premium for a stylus wasn't worth it to me for a device I wanted to use purely for reading. Obviously there's a market for such a thing so I'm not saying it's bad, just that it didn't might my own needs.
Calibre works well for the Libra 2. I have a plugin that automatically reformats epubs as kepubs (see https://wiki.mobileread.com/wiki/Kepub). That's not a hard requirement but it improves the reading experience a little bit. The biggest difference I notice is that it makes page numbers handle font size changes more accurately. If I halve the number of words that'll fit on the screen, the reader shows that the number of pages in the book has doubled. Also, the current page number increments every time I turn a page, not just when I cross an invisible page boundary.
It's not a dealbreaker not to have those things, but they're nice and worth the 30 seconds to enable in Calibre.