I would extend the functionality a bit… not a smart phone but 

Yes to e-ink (from other comment)

Mesh network connectivity 
P2P Private messaging 
P2P voice 
On chain signing 
Lightning wallet 
Cashu wallet 

100% FOSS 

Maybe extend/adapt the capabilities of this device… 👇

https://youtu.be/40llxjrIG3w?si=zf0usbhn76JbKDvZ