Since when does failing at a task change the goals of the task after the fact?
Because he is the reason he failed,he left them the chance to stop him for no reason. All his plan did is leave his loved ones at the mercy of the world without their greatest defense. And you can keep bringing up reasons as to why he did it, but it doesn't negate that he himself says "Idk lol" when asked why he started the rumbling. We've known for chapters and years that Eren wants freedom and is willing to do whatever it takes to get it, but they got cold feet at the last second and went "Only 80% of the world is dead? Thank you Eren you're not so bad after all." that combined with killing his mom, simping for Mikasa a girl who was already in love him, and leaving his friends in a bad spot is why this is bad writing. And Ymir's full circle is one of the worst aspects of the whole thing. This all happened because she was in love with the man who raped her, beat her, made her into a breeding machine, and tried to feed her to dogs? Ymir deserved better.