I cycled this route from 25 - 27 Jan 2019
250km out of 312km are tarred.
The first 30km are tarred.
Then 50km of tracks, through villages, sometimes deep sand, difficult to know which tracks to follow as people seem to drive where they like. The locals advised to follow the power lines (which lead to Khartoum).
I've marked this section in red below.
Then tar for 50km to the turn off to Shendi. Easy to miss the start of the tar. Google maps and GPS helps to find the tar. Coordinates: 17.0260825, 33.6602214 from google maps and here a picture.
From the turn off to Khartoum it's basically tar for 175km. 12km in the Sabaloka game reserve are under construction.