Yep, what @pazos said
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And also that from KOReader's PoV, anything on plain Linux is generally far superior to anything running on Android. Even very recent Android devices with gigantically overpowered CPUs on more modern Android versions are still playing catchup, and are still a PITA to handle in any decent manner, and that directly affects how the eInk screen is driven (especially in terms of latency, and also in terms of accuracy in some cases).
I'd also add that, for a new buy, I'd refrain from anything older than a Clara on the Kobo front (which essentially leaves the Clara, the Libra, and the Forma. I'd steer clear from the Nia unless you really can't spare the 10 bucks or so difference between it and the Clara, because it's at a really weird price point and generally not worth it).