Quick reply...

My friend, after hearing the idea "no display - two TOHs instead", suggests to go 'constructor' all the way.

For example: WiFi not soldered onto the board, but USB-plugged-in; doing the same with cellular modem would be bulkier, for some reason (USB cellular modems tend to be too long, in my view; antenna, maybe?), but should be possible (nano-SIMs exist for a reason).

Easy to take out microphone and speakers (it's probably over-doing it, imho, but still, easy replace-ability would help).

From outside, it should still look the same: a phone. The USB ports shall be sunk deep into the device, so that the WiFi-cellular would not dangerously protrude to outside, but would be barely noticeable.

Best wishes. Thank you.