I feel like Zuko’s not a great comparison to Darth Vader. Zuko’s arc relies on him joining the protagonists, his redemption isn’t the end of his story. This isn’t the case for Vader, and I don’t think it should’ve been either.
Yeah, Vader isn’t the sort of character where you could have him spending time with the heroes and becoming part of the group and earning their trust. That wouldn’t work with him. It’s how I might’ve redeemed Kylo Ren, but not Darth Vader. He was a shell of a man. His only option to atone was to die.
If Anakin had survived and come back with Luke, he would still have to be tried for his crimes.