Zeke did not have the coordinate power, so he needed Eren to use that power to take then to where the paths converge without being chained by the kings will. After rereading, it would appear he voluntarily waited for Eren to pretty much be 99.9% revived because he really was an instant from death, but he only waited because of sentimental reasons e.g. it's his brother, and he wants to know if he really understood Zeke, and zeke has an intense hatred and disappointment for his father that he wanted to share with Eren. Eren reverse uno'd Zeke, and now they're traveling through time/the path to see their fathers whole story.
Grisha finding the royal chamber and holding off on the attack to raise Eren was a fucking gut punch man, considering all we know about Grisha until now.