There are many works on the history of this wonderful equation. For example
Vizgin V P, Smorodinskii Ya A "From the equivalence principle to the equations of gravitation" Sov. Phys. Usp. 22 489–513 (1979)
Good job.
However, the authors overlooked a key detail of this story.
Let's try to reconstruct history from the point of view of physical ideas and their evolution.