So...I have been thinking about the mechanics of the rumbling and how all of those colossial titans would move as a group. Also how well Eren would be able to control them.
When Eren used the power of the founding Titan, he was only able to direct titans towards a target. Lets pretend that the rumbling works that way. And I'm not talking about the test rumbing that is only one district, I am talking about the real thing with all of the walls.
Lets say Eren sends all of the colossal titans to Marlay. They would obviously take the shortest rout. That is fine for those of them that are of the side of the island facing Marlay. But the Colossal titans on the far side of the island would cut across Eldian territory to get to Marlay. Eren doesn't have royal blood like Zeak. There is no reason to believe that he can guide them all around peoples homes. They could be the first to be flattened.
If they are smart, they continue to release the colossal titans one at a time (or in batches) so they can be directed more easily. But Eren seems to be in a 'Do first think later' kind of mood right now.