This is one of the major flaws of Isekai. A lot of them deal with slavery in one form or another. Some of them try to eliminate slavery. Some, like this one, embrace it.
There's no reason in the story for the people MC interacts with to be slaves. The author is using 'Slave' to mean, 'it's not just that these people won't betray me, they can't. The magic prevents them from betraying me.' Even if the author wants a magical ceremony of some kind for this, a marriage ceremony works just as well.
I love this story, but this is one of the big negatives to it.