In the Kobo forum there is a
thread complaining of there not being enough contrast between the screen and the font on an Aura ONE, so be warned. (Edit: The thread is about the Aura HD, not the Aura ONE.)
If you can find one, try a Glo HD. Another option is the Nook Glowlight Plus. Both are 6" devices that handle ePub. Both have 300ppi Carta screens. Neither has expandable storage. The Kobo has a better UI and is more hackable. The Nook is waterproof.
There's also the Aura H20. Screen is 6.8", has expandable storage and is waterproof. That may be your best bet.
Question: Did you try turning on the light, even in a bright room? The light can add more contrast to the screen.