I have a similar shopping list and very little that satisfies it.
General commentary:
- A laptop or e-ink tablet can run other voice comms (Jitsi, Zoom, Skype, etc.) These are also generally more capable systems and convenient to use when seated or established at some location. The dividing line between present-gen phones and smaller tablets (beginning at 6") is ... slim.
- SMS is itself not secure and highly problematic. As much as I'd like a dumb phone, I'm not sure I'd trust it to even SMS. Voice comms alone, or support for a secure encrypted messaging system. SMS itself seems too limited to allow an encryption extension, though xmms might suffice.
- Splitting the phone and the hotspot functionalities is an option. That's an additional service, but might be preferable.
- For the phone, I'd prefer a monochrome display, preferably e-ink, with a backlight or frontlight for low-light conditions. Hardware keyboard similar to the Blackberry, Palm Treo, or Palm Centrol might suffice.
- The Psion Series 5 may well have been the perfect mobile communicator. https://en.wikipedia.org/wiki/Psion_Series_5
Otherwise, a B&W candy-bar phone along the lines of the Nokia 3310, or a flip phone like the Ericsson T28 or Motorola Razr would be a welcome sight.
The feature I'd most appreciate in any of these for the present which wasn't available in their initial deployments is strict whitelist-based call acceptance. Any unknown numbers roll directly to voicemail or are rejected entirely.