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.