There's a fine balance to find but you can not that hardly still find a way to have a slow corruption. You can have him, if I take that example again, believe that he's finally found the woman he was searching for and so he doesn't just want to fuck her but to own her, so he takes things a bit slowly. I don't know, but not what we got (once again, it's my own humble opinion).
That is what we got though? Brutus can and does know he can break women just by repeatedly having sex with them until they give in (because porn/hentai logic). It's how he's explicitly been portrayed up until the reveal. It's what he does with Alys.
If he wanted to, he could've done that with Cyanna by blackmailing her with Leto's safety and going straight to sex. But he doesn't want to because A) he wants to preserve his fantasy of Cyanna actually falling for him of her own accord, and B) Brutus does want to "own" her, he just thinks that desire is "love" because he's been twisted by his past.
It's in the same way stalkers and abusers think they "love" their victims, when they just want to own them. If you genuinely love a person, you don't blackmail them into having sex with you.
I feel like the best NTR antags are the ones who, presented with the unexpected death of one of their girls, would just shrug it off saying "Ah, fuck... One less toy to play with, that sucks... Well whatever there are always new ones".
That's fair. I think that it comes down to more of a personal taste then. I do agree in some aspects to you, I think the tavern keeper would've been a more interesting NTR "pair" for Lily compared to the shopkeeper. But thems the breaks.
For me, one of the things I don't really like about NTR is that the girls sometimes fall too easily, in my opinion. It's fine in some stories, but in stories where the girl characters are built up as strong people, the impact of the corruption is greatly lessened by the fact that they betray the people they love after like, a week of fucking people.