I need to keep the heat production down, way down, for the Australian summer. My Pixel powers off if I leave it on the passenger seat while I drive with aircon on, and its only late Spring. So that's a design constraint you probably won't have.

There are a bunch of dubious diy smartphones built around the RPi Zero, but the heat production is unacceptable for me, even though I could fix the hideous form factor.

So far I'm looking at hardware:

Cellular module (anything SIM7000-compatible should do)
e-Ink screen (4.3")
Mechanical i2c keyboard
LoRa module
Huge lipo pouch cell (slow boat waiting on for two months now)
1S BMS
Raspberry Pi Pico W
Remote server (or shell account) to run heavy apps

For the software, I've been collecting libraries. 

Going to run Micropython and go bare metal for the native apps (sms, calls, email, terminal client, image viewer, TCP-IP and Reticulum) and run as much as possible on the server.

Will see!