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