Are physical keyboards a must? What about something like a Fairphone running Ubuntu Touch?
Or, for that matter, what is driving the need for Linux? Could it be satiated with an android device and/or x2go to a self- or cloud-hosted Linux desktop VM/VPS
I would suggest something compatible with Ubuntu Touch and a separate portable keyboard. There are some cool universal/bluetooth ones out there…foldable, compact, with device holder, etc.