I wonder if the generic tablets with made up Asian sounding names that you see on the big marketplaces could work? They could have slightly larger or smaller screens if they’re cheaper than current phone screens, and could have Linux with something like Signal on them for calling.

They’d probably be lower quality to begin with, but could potentially get better if people start to buy them. They seem to have generic hardware, so might be able to offer the drivers and unlockable bootloaders too :)