Honestly, this is probably one of the best Isekais out there. It's harem and all, sure, but it's tastefully done, the MC isn't a pathetic little bitch and it kinda makes sense.
The only small grief I have with it is that Roxanne doesn't seem to know she's in control of herself. It looks like she believe she's doing it half because she's forced to, half because she wants to.
Like she's got that "He is my master, not my lover, I have to submit even if I like it." idea, while the guy, in his mind, he's convinced Roxanne is giving herself to him out of her own free will.
It would be great if the Author addresses this. Have the MC unequivocally tell her she's her own master of her own free will or outright free her as a slave. That her being free doesn't mean she has to go.
Apart from that? Yeah, it's a good manga.