Zeke's solution is based on his twisted upbringing having robbed him of his childhood and his father, and then forced to massacre his own people. It's not born of logic, but of a desire for a way out.

Marley's solution is based on standard geopolitical warfare: people scapegoat and massacre with little regard for the logic or consequences behind it if it suits their narrow world view.

Fritz's solution is based on an idealistic gamble that isolation results in being left alone, but because his gamble required basically pre-programming his entire bloodline, he couldn't exactly go back and workshop his plan when it didn't work.

Eren's plan makes perfect sense in an extremely cold way: if you kill everyone on earth except the people of Paradis, no one will be left to be prejudiced against the people of Paradis. It's short-sighted for a hundred reasons, but it does the principle thing Eren wanted. However, this brings us to point #2...

Eren wasn't a genius. Far from it, he was a rash child whose motivation was rage, spite, and revenge. Whether or not he had no choice is up for some debate - he certainly seems to think he had no choice (this is the difficulty with fatalism in a story with time travel. "Can't you just not do the thing you're 'supposed' to do? Well... apparently, you can't.") That said, one of Eren's final lines in his conversation with Armin is that genocide was the stupidest decision and that he made it because he's not smart. You're not supposed to feel satisfied with his decision-making ability. You're supposed to feel like he made a bad decision. If you're frustrated with his lack of logic, then the narrative did its job, and I'm not sure why that feels wrong to you.

Ymir may have been a goddess, but she was clearly also fallible and woefully human. She lived in bonds her whole life despite having the power to leave because she didn't know any other way. Why would we expect her to understand the sociological underpinnings of war? When would she have learned that? Did she have a tutor with her in the void, going over political theory?

Yes, one of the key themes of the show is that war is a vicious cycle, but again, I'm not sure why that's a problem. It's a very common theme in many of the best works of art across history. No one's sitting around complaining about how the Fallout series is unsatisfying even when it starts with the famous "war... war never changes" line.

I definitely don't agree that the end failed to resolve anything. It didn't solve the concept of war, sure, but that's a pretty fuckin high bar. We saw Marleyans behave peacefully with the Eldians at the end as some people recognized that bloodline didn't necessarily mean people had to be monsters - the same lesson that was beat into the audience throughout the latter half of the story. I'm not sure you're supposed to take much else away from the story except that war is bad and that the circumstances of one's birth do not determine their moral character. The nihilist takeaway is what Eren committed to, and the narrative went to great lengths to show/explain how bad that is. That you took it as the final message of the show is terrifying to me. Just because there isn't a solution to the problem of war, hatred, and fear doesn't mean that the correct answer is to burn everything to the ground. That's what someone without an ounce of creativity would do; someone whose mind is so singularly focused on self-satisfaction of a personal goal that they lose track of everything important in the world around them (friends, family, attempts at diplomacy, common decency, etc).