Why didn't Rod Reiss just eat Eren? Why did he need Historia? Why didn't Rod Reiss just eat Eren? Why did he need Historia?
If Reiss just ate Eren, he would be the king, and do whatever he wanted (except that the King's will may overrule some his own free-will).

In the Anime, he even says that "I must not become a titan myself".
src: Attack on Titan Season 3 Episode 44 – "Wish" @ 13:46
In an interview included in the guidebook Attack on Titan ANSWERS,
  Hajime Isayama revealed the reason why Rod said he did not inherit the
  Founding Titan himself: "He thought of himself as just an observer,
  and in order to preserve the Reiss bloodline, he may have thought that
  he needed to stay human and make more children. He would have to
  become the Founding Titan, and he was most likely afraid that he
  wouldn't be able to fight against the First King's ideology, just like
  his father and Uri weren't able to."


src: http://attackontitan.wikia.com/wiki/Rod_Reiss