I’m surprised to read about the bad battery life of the Oasis. I haven’t read this before. Which model is it the Oasis 2nd (7”) without adjustable warm light or the 3th/present Oasis?
I would only recommend the kobo sage for reading by buying the PowerCover also.
Without the PowerCover, the Sage's battery won't last long. The battery life of the Sage is poor.
I had the Sage and sold it. However, the reason wasn’t the bad battery life, the Sage didn’t fulfill my Note Taking demands. The display and typeface on the Sage were really good.
The Libra 2 would be the much better choice with his long lasting battery life, perfect display and contrast. I have the Libra 2 as back-up Reader to my Oasis 3, 32 GB.
Another device that might be interesting for you is the PocketBook ERA. Like the Oasis the ERA have a flush screen. It is available with 64 GB. Maybe you will get more and more books in the future, 64 GB fits better than 32 GB
I was thinking about selling my ERA, 64 GB and had already advertised it. But since it is more handy and lighter than the Libra 2, it will be the e-book reader that I will take with me on the road.
One point is, with the ERA I would recommend to use the device with KOReader cause the setting options of the PB software are quite little.