Maybe I'm crazy but I think we're supposed to read this story with the understanding that slavery is bad, this society is bad in practicing it, and Halo is a fucking idiot and immoral for just rolling with it. We aren't supposed to uncritically side with Espada, here. I mean she had the line "There is no such thing as 'self-defense' for monsters." That's crazy. Halo goes along with this because it gets her what she wants, right now; she didn't even stop to think about the wider ramifications of a perfected slave spell.
Espada wants to tame priority-one threat monsters because of all the good she thinks that could do. She wants Halo to develop this spell and to give it to the Guild; she trusts her higher-ups in the Guild to use it properly. Is that because she doesn't think the Guild will let it get out to slave traders? Or because she didn't stop to think about the ramifications beyond what she's focused on? Or because she just doesn't think that much about people being enslaved because she's from a culture where that is normalized and she isn't herself enslaved? I don't know. For that matter, I'd be at least a little suspect of what Espada imagines the Guild might do with their subjugated mega-monsters, once they had them. She has done a lot of killing.
Halo is a typical isekai protagonist in that she just doesn't really think all that much about cultural differences or question the morality of the systems she's a part of, she just happily goes along with things and focusing on her own short-term happiness; the difference is that the story portrays her as an idiot and a bad person for it, if bad in a lighthearted comedic gets-her-comeuppance way via getting slimed or spiking herself with aphrodisiac. All of our other main characters have things with their stories that put this world's society in a negative light, and Halo stumbles into being a helpful figure and someone they can become friends (family? lovers??) with. I think this is going somewhere other than "And then Halo made the perfect slavery spell and everything was fine hooray". Maybe Filia will have thoughts about Halo working on that when she finds out!
It's a fantasy setting, and it's very much a fantasy setting, it's quite different from real world historical periods; but I think it does a good job of showing bad things about the world in a largely realistic light?
Another thing on the slave spell, unless I'm forgetting something huge, Filia is still under it. Halo thinks to herself she could break it with little effort but doesn't actually break it. Halo just sorta stopped treating her as a slave, at first because she dug herself into a lie, and then because she's just been seeing her relationship to Filia differently. She didn't really ever think of her as an object, but as a person. A person who conveniently would have to fuck her, but a person. She doesn't really seem to grasp how monstrous a thing slavery is, and after learning how horribly Filia suffered... Didn't really internalize that, at all.
Halo objectifies people, mainly other women, without thinking much of it—without really considering what it is, exactly, she's doing. She doesn't really seem to want power over others? Maybe she has some inclinations to certain forms of sadism, like with the slime chapter, but mostly... I mean... She wanted a sex slave so she didn't have to say out loud that she wanted to have sex with a woman. That's explicit from chapter one! She is a really immature person. I think she's a good deconstruction of the isekai protagonists who have slaves to gratify the reader fantasy of easily available, assured sex; by just having her be like that but then still folding to social pressure and embarrassment and, when things are blatant enough, her own morality... and so still not having had any sex! It's simple, but I think it works.
Also oh man so volume six is the last one, then? On the one hand that's a shame, on the other I'm excited by the thought of seeing this come to a finish. I really want to see the note this ends on. Will it stick the landing, and be so much better than it has any right to be to the end? Or am I just delusional? Or does the manga end a million years before the original web novel and so it's not actually an ending? Hahah, let me pretend it's definitely not going to be that one, please, please.