I'm not much help. I've stuck with 6"screens. But the Libra 2 seems to be one of the best readers Kobo has ever put out. It seems to garner a lot of praise.
Do Kindles need to be jailbroken to install KOReader? If so, be aware you may not be able to do that.
KOReader on a Kobo is no biggie. On the other hand, the things you liked: collection management, and to display the current book cover are things that will work in regular Kobo Nickle firmware out of the box.
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