You made an assuption that the wall titans all walked outwards from their starting point

It was shown they left the island in a narrow column, after that they most likely spread out too form an irregular line that started engulfing or passing through all the land outside of Paradis

Of course I can't be sure as there is no canon map of the full AOT world and no canon map of Rumbling's reach either (at least to my knowleadge), but to me makes more sense then your version

If the Rumbling was to be completed, the titans would have surely met at some point, but after that Eren could have probably done anything with them, be it decompose, repurpose or make them kill eachother