KOReader sounds cool but I've been happy with the built-in software. I know me, and I specifically don't want other knobs I could be playing with instead of just reading the thing.
It seems it's between two devices, the Libra 2 and the Sage, but I keep hearing the Sage has worse battery life (sort of the downfall of the Oasis for me as well). I just like using Calibre since I get a lot of epubs from places like HumbleBundle and other book bundles. These always have issues with the Kindle, like covers going missing.
I'd love to get something a little less fickle for my use case and it does sound like the Kobo devices might get me there. I just really like the Oasis from a materials, size and general usage.
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.
But that gives me time to save a few buckets as well. For now the Oasis is working well enough, just it'd be nice to get away from some of the paper cuts.