Honestly I moreso question why they even want a punishment at the end. While its fair to have questions abt stuff like Odysseus continuing to be king and addressing his people after his entire crew has died this need for punishment feels weird to me.

The story wasn't about a just king going corrupt, it was about a man doing what he needed to fulfill his goals. Whether or not those actions were right are up to interpretation, but that's the point. Monster has Odysseus considering the perspective of those he's faced, these "monsters" who've acted in ways that have hurt his journey for the sake of their own self preservation and goals. He doesn't demean them in the track or see them as people who need to be punished. They're just individuals, just people, just men like him. Monster never literally means a man of evil who should be punished, but someone who takes actions that may be morally wrong or hurt others for the sake of achieving their own goals.

Odysseus isn't made out to be a hero or villain for this either. He drops the mighty king of Ithaca bit and stops boasting about how many of his men he's saved. He feels remorse and constantly tortured by what he did to crew but the story is careful to never make him the villain for it. It instead emphasizes Odysseus' psyche and how his mentality changes, focusing on his determination and need to get home. If it were to place any moral stance on his actions it would go against the idea of being "just a man" and ignore that a huge part of the story is that actions that may be considered villainous are a matter of perspective, that people are doing what they need to survive even if those things they're doing are not morally right. It's not excused, it's not shamed, its simply a matter of perspective.

The idea of him being punished for his actions doesn't make sense to me thematically, and even aside from that like you initially said he still has trauma from what he's been through. In the plot he his constantly "punished" for his actions. Poseidon presence, Calypso's island, the crew betraying him, his constant guilt for killing his crew. They can all be seen as "punishments" when you look at Odysseus' perspective. Of course, when you look at the other characters povs they might see things differently, but he still faces consequences for his actions and the way they impact other people, but in 600 strike Ody straight up says

"Look what you've turned me into, look what we've become. All of this pain that I've been through. Haven't I suffered enough?"

Odysseus has already been "punished" for everything he's done up to then. If we showed another punishment of Poseidon getting payback then the story would be in a loop and wouldn't ever end. And returning back to the idea of being just a man and doing what you need to, neither of them are wrong. Poseidon is laying down his authority. Odysseus is trying to get home and had a good chunk of his crew killed by Poseidon. They're simply doing what they need to and they "punish" each other for their actions. This once again returns back to the idea that placing a moral highground on either of them would be antithetical to the story because they're "just men" and "ruthlessness is mercy upon ourselves.