I suggest making Nejd (2W, 1N. from the camels in eastern Arabia) a hill terrain to represent the plateau (since mountains are too much at this scale), and add 1 oasis north of it Nejd hill to represent the large oases in Qasim, as well as turning the terrain under al-Hasa (oasis with dates) to a more type that has higher food yield, as these were historic agricultural centers relative to the general aridness of the desert.
On a side note, I like how it is possible to squeeze Medina into the hejaz region north of Mecca.