How different would things have been if House Justman hadn’t been wiped out by the Ironborn?
Very, very different. While still making a lot of the same mistakes w/r/t city charters as other regimes, the Justmans were probably the strongest of the Riverlands dynasties.

If they hadn’t died out, then:
- Right off the bat, the Riverlands avoid a hundred years of bloody civil war which restarts the Bracken-Blackwood feud.
- They avoid the instability under House Teague, and the eventual religious civil war that brings about the conquest of the Riverlands by the Stormlands.
- As a result, the Stormlander empire in the Riverlands never gets off the ground. This has a lot of ripple effects: first, the Riverlands is spared ~360 years of rebellions and repressions and having to fight in Stormlander wars. Second, the Durrandons don’t get over-extended and probably stand a better chance against the Gardeners and Martells.
- Next, the Hoares aren’t going to be conquering squat. 3,000 men against 35-55,000 isn’t even a concern. Not only does this spare the Riverlands 140 years of really brutal rule, but it also means the Ironborn mythos of superiority isn’t going to get the shot in the arm it gets in OTL. Maybe Qhorwyn’s New Way gets a chance to sink in and the Ironborn continue as a nation of traders and strictly overseas pirates/mercenaries.
Now what happens when the Targaryens come, I don’t know. But if the Justmans survive, they’re going to be much stronger and more stable than the Tullys, that’s for sure.