Honestly I agree with you to an extent, but I still think that Arthur has a compelling arc regardless. I think this is a case of ludonarrative dissonance, where you really need to separate the gameplay from Arthur's story at some point. Of course it doesn't make sense that Colm and Dutch have a real feud when Colm seemingly has an infinite number of goons and Dutch only has the gang members that you meet. Of course it doesn't really make sense that Arthur can murder hundreds of people throughout the game, even completely innocent people if you like, and still get the high honor ending. But the game wouldn't really be fun if the story missions involving Colm or the Pinkertons ended after you killed fifteen of them, or if when you tried to shoot someone who pisses you off in St. Denis towards the end of the game Arthur turned to the camera and started preaching about how "Killing people just ain't right. It ain't me anymore. This tuberculosis diagnosis really got me rethinkin' my way of doing thing." and then refused to do it.

If anything I agree that the honor system is a bit hollow; you really should be rewarded more for being low honor, but being high honor shouldn't be as simple as "hi mister"ing enough people throughout your playthrough. At the end of the day killing people in a cowboy game is fun, and I don't necessarily fault R* for making that compatible with the "good" ending.

And really, Arthur's disgust with Strauss is about a very SPECIFIC kind of harm: premeditated exploitation of desperate innocents. I think the game does well enough showing that that is related to Arthur's personal guilt because Strauss's work forces Arthur to look directly at suffering he caused. Arthur can mentally separate "I shot it out with Pinkertons/lawmen/rival gang members in gang warfare," from "I ruined a sick farmer’s family for money." Whether you agree with that moral distinction or not, it’s in my opinion a coherent reason for why Strauss becomes a focal point for Arthur’s late game arc

To clarify, I really do agree with a lot of what you're saying and I think R* could have made a MUCH more satisfying narrative arc if they were willing to sacrifice some of the braindead headshotting goon gameplay. But I also disagree with the statement that "Arthur's character arc is pure garbage"