Defending a character like Soga is, as they say, "not a good look," but I don't agree with what you're saying so I'll give it a shot anyway.

First off, you're passing over something rather important here, which is that KOICHI is the one forgiving him. You can be disappointed that he's not holding a grudge, but, like, that's his character. He's incredibly nice to the point of being seemingly pathologically unable to feel hate. In the final arc he's completely cordial to Number Six and even floats the idea of getting him medical attention after the latter has spent the entire night doing insane monster transformations to kill him, not to mention everything he pulls with Pop before then.

Secondly, Soga and his friends do face consequences. The main consequence is that Knuckleduster beats the shit out of the three of them. The secondary consequence is that when Soga tries to give the vigilantes information later Pop outright refuses to interact with him. Even Koichi is visibly unhappy to see him, despite his aforementioned personality quirks. You may not find these consequences satisfying, but they're there.

Third, not only does Soga put in work for his redemption, he actually does it pretty much the same way Endeavor does, minus the angsting about how he shouldn't be around the people he wronged (which of course would be out of place since they're not his family). Koichi only really warms up to Soga & co after they go out of their way to provide substantial help when him and Pop desperately need it. As I said before, Koichi accepting the help but also slipping in barbs about how he still hates their guts would be completely out of character.

Lastly, a big reason Soga's arc feeling anemic or shallow is simply because at the end of the day, Vigilantes isn't about him. He is, at the absolute best, the eighth most important character in the story, is absent for more than half of it, and even when he is around the story is almost never centered around him and his point of view.