There is clearly a limit to how much the body can take when transforming frequently.. whether the stamina required to resist it is something that wanes with time or you reach a point of no return with the damage done, I cannot say for sure.

Let's ask him about it. Maybe he knows more than we do.. there's a lot he didn't talk about it the last couple of years..

[This is less about what he had been planning, but more about everything else. There is the possibility that Eren knew more about the titan powers than he mentioned, or perhaps he knows specifically the steps to their demise and never thought to mention it seeing as it is deemed inevitable.]

[Asking also means that the two of them can easily slide into the 'find a way to let you live past the thirteen years' thing that they have going here. Hange is certain that Eren would be up for trying, but it is his life after all so it wouldn't do to go behind his back on such an important matter. He needs to be involved!]