Your idea to start them off in Galt sounds interesting. It reminds me of the spanish civil war. There were so many different groups involved. Catholics, monarchists, facists, traditionalists, communists, socialists, anarchists, republicans...
Many left-wing idealists from across Europe (including George Orwell, see Homage to Catalonia) went to Spain to fight against facism, only to get mired in the squabbling between different subdivisions of communist.
The party could easily be idealists, locals, or just people caught up in the conflict and "encouraged" to participate. You could ask your players for thoughts about how their character might be motivated to be involved. The old family vendetta or working for coin are always available.
Some guerilla fighting, "get the intelligence" type heists and betrayal by former comrades give you some scenarios to string together and provide some potential fodder for the hasbeen campaign. You could end the scenario with the party working their way out of Galt after being betrayed or becoming disolutioned.
It doesn't have to be dystopian, because the rest of Golarion is not suffering the chaos of Galt. That gives hope of escape, or hope Galt can be saved. Especially when the revolution seems to be going so well in Andoran. You can even find in game ways of letting them know how well things in Andoran are going to serve as method of drawing contrast with Galt.