I have just picked this up and it feels like a well written train wreck. It's looking like a what a scums wish should have been. Not my thing either way but as train wrecks happen, I can never seem to look away.
I can always respect and author that's willing to do justice to this kinda story.
Scum's Wish teased a lot of interesting relationships, only to never committed to any of them, and end up with the most vanilla, most bland thing possible. Something mainstream manga authors seemingly love to do. They tease the big "forbidden" themes, sometimes even in the title, and then of course never actually commit to any of them.
Brides of Iberis at least properly commits to its themes, but given that, this was written by the same author as Octave, it had a snowball's chance in hell for a good ending. And that's ok. The ending perfectly suits the themes of the series, and it's how realistically a story like this would usually end.
But I always felt that trying to make and ending "realistic" often get in the way of making it "great". There are no endings in real life. No definite end points, no fast forward 10 years, no happily ever afters. And the unexpected does happen all the time. So I always felt, that even for a manga cemented in realism, the ending was the one place, where that restraint should be a lifted slightly. To make an ending that is more than "all as expected", to end the series on a high note. It doesn't have to be a happily ever after (and in fact it shouldn't), just allow yourself to make and ending that is merely plasuble, but noth the most common.
All is to say, yeah totally wanted them to end up together, I kind of have a thing for morally fallen characters, who instead of taking the moral high ground (like Mitsuki did), end up on a morally questionable path, and stay there.