Look into the PinePhone, it runs Linux and there are several distros. You can open the back of the phone and even swap it out with other backs that add function

Look into the PinePhone, it runs Linux and there are several distros. You can open the back of the phone and even swap it out with other backs that add functionality. They're working on a keyboard that looks a lot like the old Psion Series 5 keyboard. It replaces the back of the phone and turns it into a clamshell style device.