[chasil]

Buy something that runs the latest LineageOS, and use the Kindle app.

If you want greater security, substitute Graphene for Lineage.

These will not be e-ink displays, but the longevity is perhaps the longest available from independent vendors.

[fsflover]

If you choose non-e-ink displays, than the best longevity will be for GNU/Linux devices like Librem 11.

[fmajid]

They likely won't support the Kindle app, however, and the users won't be able to access the books they paid for but don't really own thanks to DRM.