Realistically, the rumbling as a deterrent is the best option. To make demonstration that makes the world stand back from Paradis.
The thing is, Eren's current state of the mind is a mystery.
He made a vow in front of Reiner to "destroy all his enemies until the end". Can we trust Eren to really be rational and expect him to not unleash the rumbling all over the world? Does he want Paradis to be left alone? Does he want to restore the Eldia Empire? Does his definition of freedom mean only Eldians living and the other races annihilated?
While I understand Eren is motivated into freeing his people, I can't help but being annoyed of how Zeke and Yelena undermined all possible paths for Paradis to take once they learned the truth. Paradis had a 4 year break from any Marley incursion save for the survey ships, but there very moment the first ship with Yelena arrived, Zeke essentially took control of Paradis.
Rather than allow Paradis to deal with the Marleryan prisoners by itself, which could had established the first formal contact with Marley's government, Yelena took control of the narrative, ensuring no communication/negotiations with Marley at all and making Paradis establish contact with a nation that sabotaged their diplomatic efforts with other countries. Yes, Marley declared war against Paradis, but there was 4 years of radio silence from the island except for the fact all their ships never returned. Yelena and Zeke ensured that the narrative of Paradis being threat to Marley remained and ensured the declaration of war would happen after their war with the Middle-east union was wrapped up.
In addition, was the Liberio massacre really necessary? Wasn't it possible to perform a more discrete operation to sneak Zeke out of Marley? The massacre felt more like a maneuver to force Paradis into the rumbling plan, rather than a act first strike self-defense.
Don't get me wrong. Without Zeke and Yelena geopolitical's input, it is possible that Paradis could have made a series of amateur and disastrous diplomatic mistakes, but it seems to me that Paradis itself has been more like a pawn if Zeke's plan, who doesn't give a shit about the nation, and is more concerned with his race itself and Paradis never had a chance to be it's own nation. Because of Eren and Zeke, Armin and Hange's plans of diplomacy were sabotaged from the very beginning.
It is a complicated situation and it is far from being a good vs evil scenario. Which is why I love this series so much.
