An enlightening video from Mentour Pilot (he has now deleted the video from his channel because he doesn't want to speculate) that shows them trying a similar situation in a simulator.
https://vimeo.com/329558134
At the end their stab trim is at 2.5 units (0= full AND, 10= full ANU) and it is extremely hard to manually trim it back. The Ethiopian plane had about 2.3 units of stab trim for most of the trouble period before going close to 0 (full AND) at the end.