Uh, so he actually can't publish classified data. Many, many lawyers would have reviewed that article to make sure he wasn't breaking the law. Because, yes, everyone in that chat group was breaking the law, but the reporter is the one who would've been prosecuted and "But they did it too!" isn't actually a good legal defense.
The information he refused to publish was detailed operational information, such as sequencing of American fighter jet attacks. Aside from the very emphatic treason charges he would've been slapped with, he doesn't want to put the CIA officers or troops in danger by publishing the information that could get them killed. And publishing that information guarantees foreign governments get it, not just the highly likely instance that they get it because government officials were talking about it in a Signal chat.
Lastly, yeah, he left the group chat. He was going to get dropped once he published anyway, so he left it as soon as he had confirmation it was real. How long should he have stayed? What number of deaths would have been required to make it "acceptable" for him to have left? Should he have stayed all 4 years and published it in a book after?
"Informed them of their mistake." I guess the previous one wasn't lastly, this one is. He reached out for comment before publishing the article. That's what reporters do. At an institution like The Atlantic, they're always going to do that. And at this point, he knows that asking for comment isn't going to lead to them covering anything up because they're either too stupid or too careless to cover it up.
In conclusion, he actually did a lot right. (I won't say he did everything right because I wasn't there, but it sure seems like he did.) This is the Watergate tapes. We just don't have anyone in government right now willing to act on it in the way Congress was 52 years ago.