In terms of hardware, re-use the SoC from the Librem5, since it would already have mainline support, cut out all the cellular stuff, like 4G modem, GPS, Bluetooth etc. to reduce the cost of the finalized PCB and speed up development time significantly.
On the software front, make a minimal Linux-based firmware, capable of booting https://github.com/koreader/koreader as the GUI.
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