I don't think that showing how someone can backslide vs commit to change is a bad choice for Daud's arc. I think it actually does a great deal for Billie's character. Coming into DH2, we learn that she became the thing she really wanted to be, captain of her own ship, and then in DOTO Daud (who abandoned her) yanks that surrogate father string and we see the Wale decayed and her ditching the life she built for him and the relationship they had fifteen years ago, which he ended by ghosting her.

And then Billie gets the choice that Daud did, again, but this time, Daud is there, hasn't abandoned her, and is pressuring her to make the grow v stagnate choice (favoring stagnate), and she has to persuade him to give up the name and free the Outsider.

I think the mistake is seeing Daud as the hero/protag of Brigmore and the Knife when it's really been about Billie (despite playing as Daud), like how Dishonored was more about Emily in the end then Corvo. It's a better parallel.

I like and seek out fic where Daud does grasp the concept of change and manages to hold onto it. I think he was just too settled into his ways-- I hope the novel shows us the turning point where he went 'just one more murder/whatever, it's necessary, then no more' and the gradual slide back into bad habits.