It is interesting and refreshing to see a powerful isekai character stick to real-world values (in regards to slavery), especially when they have the power and influence to do so. I know that "buying a female slave" is a power fantasy and sexual fantasy for many young men, but stories that defy that expectation somewhat are nice.
Granted, this story still has the whole "we're not really slaves because we choose to serve our master" thing going on, but he also doesn't take advantage of them because of that relationship, which is also pretty nice.
Compared to stories like that "500 year old sage" one where he goes along with the host kingdom's expectations that "we want her bloodline in our kingdom, so we'll strongarm her into servitude to the kingdom", especially when there's like three other Japanese isekaid kids that just go along with it and see no issue with such treatment, it is interesting to see isekai characters have their morality taken seriously.