He gets a free pass

There's evidence he was trying to prevent worse things from happening. He hid the last dragons, he joined a extremist group, he was WAY ahead of Azula capturing him, his prison escape plan was likely a long term thought process he already had. Iroh was never going to be fire lord.

He was first in line to be Fire Lord until his son died.

I don't think Iroh would have been as ruthless as his brother. But he did see conquest as a sort of duty.

But then his son died, and he realized that it was all pointless.

No he "got away" with decades of war crimes by becoming disillusioned of the society he was born and bred in and becoming a political enemy of the state and putting his physical wellbeing at risk to free the people his country had waged war on all while mentoring the person who would take over that country and try to create a better country and a better world. One of the most poignant moments is when he says he didn't realize his visions of conquering ba sing se would be him taking it back for its own people.

The Mikhail Bakunin of Nickelodeon cartoons.
Which is why this is fictional, and he's allowed to have a narrative story arc.

However, if this was a Nazi SS Officer, who fled to South America, and then went on to redeem himself by [insert narratively compelling redemption story], he'd still be a war criminal.

But again, it's a cartoon, and we don't have to treat his character as if he were an actual Imperial General commanding troops during wars of conquest, especially one from the IJA.

Varrick bombed buildings and tried to kidnap a president to start a war for profit and got away with it too.

People change, we should all learn to see people for who they are, not who they were.

I'm not sure siege is considered a war crime. Isn't that just standard medieval warfare?

Depends on the culture The Japanese viewed siege tactics as cowardly and armies at the gates would deliver food and supplies to the people in the walls. Ba Sing Se was able to convince it's citizens there wasn't even a war going on, I don't think they were starving or being killed with siege weapons.
No, it's not a war crime that I can find, however we can attribute harm caused to civilians through these actions, such as starvation due to supply lines cut off. So he did some vile shit, had a moment where he realized the error in his ways, then did everything in his power to make things better.
- Except there no evidence of starvation in Ba Sing Se. After all, there was so little impact that the citizens could be convinced that there was no war.
  
  As to supply lines, earth benders cannot be locked in by a siege. They can create tunnel networks with a literal wave of the hand.
  
  So you're inventing crimes that didn't exist.
The other nations wouldn't see it that way.

There's no central authority, but it's not a machiavellian free-for-all like medeival Europe. The rest of the world was rather unhappy with the Fire Nation's aggression, even in light if the world's long history of warfare. He would be tried for that, no doubt.

And his reputation/nickname is subtext for crimes he did commit but that the cartoon couldn't spell out.
- I mean, that's essentially saying it's a war crime to be in the military during a war. Which is kind of silly to put it like that.
I mean really the existence of war crimes relies on the existence of treaties between the nations defining what those crimes are. Gonna guess the Fire Nation was not a signatory.
- The Nazis weren't signatories to the Nuremberg charter, yet they were judged by it. So there is precedent for judging war crimes without pre-existing law.

Well he does leave the army and help the opposition so I'd say he redeemed himself hard

A war crime according to... Who? Is there some treaty or convention that happened? Is there some customary international law that he violated? I can't find the Hague anywhere on any maps in this universe but maybe I missed something.