It started out like it was expanding upon his arc from TFA (what was at least coherent) but then it all goes wibbly and starts chasing it's own tail. And after all of that, what did he learn?
First he learned there were things worth fighting against, which is what he saw on Canto Bight, and then Rose interrupting his suicide run taught him that that's different from having something to fight for (which she
literally says in so many words). He doesn't fully assimilate that last bit by the end of the movie, but that's fine, because that heavily overlaps with Poe's arc, and we need to see that he's gotten the message more than we need it for Finn. Ideally, Finn would either develop or exhibit an appreciation for building the future rather than merely tearing down injustice in the next movie, but TLJ left them nowhere to go.
He went from being, let's be nice, a conscientious objector who liked exactly one person in all of creation, to someone that recognized that whether or not he could take down the First Order (or the snobs on Canto Bight), it was still morally worthwhile to stand up to them and declare himself in opposition to them.
Is it that war profiteering & slavery = "bad"? Or "don't trust thieves you meet in prison"?
That's a very superficial understanding of DJ's character as just a subversion of the "rogue with a heart of gold" archetype of Han and Lando. His dramatic purpose is far more straightforward. DJ tells Finn that there are no good guys or bad guys, it's just idiots fighting over nonsense while other idiots make money off of them. "They kill you today, you kill them tomorrow. Don't join." In the parlance of the time, it's written on his hat. Finn, about to be die for convictions he's only just found out exist at all, tells DJ he's wrong, ideas and people are worth fighting for, there's a difference between the Resistance and First Order beyond the color of their clothes and whether their ships are shaped like an "H" or an "X." And DJ, not giving a shit, just says, "Maybe" and walks off with a shrug. DJ uses cynicism as a substitute for wisdom, like so many people in the world.
Then Finn faces off against Phasma, the symbol of his oppression, declares himself to have joined in defiance of DJ's advice ("You were always scum."
"Rebel scum"), but he's still not self-actualized. He has meaning, but it comes from rebellion, opposition to the bad rather than devotion to the good, which is a common Star Wars problem. In "The Mandalorian," we've got another character who hates the Empire, but when they're gone, she's got nothing left for the world without them. Rose is trying to protect Finn from that.
