I have complains.
1) The "genius strategist" title is undeserved, the author simply dropped it on us without showing anything. I would already be surprised if a modern zoomer knows how to tie his shoes. Conquering a city is very likely out of question.
2) Monsters and humans can't coexist for a reason. Trying to force them is probably a foolish idea.
3) A captain leading a siege can't decide or promise anything to the conquered humans, because operating a city requires resources and personnel that the MC probably doesn't have. I assume the MC decided everything on his own because of the reaction of the big-boobed chick.
4) The benefit of sparing people's lives and not even enslaving them is absolutely insignificant compared to the immense threat of having enemies within the defensive line. The people lost their loved ones, if not from the siege, then from being a border town. I guarantee that the majority of them dream of turning the orcs into kebabs and no amount of propaganda will change that.
5) The MC can leave anytime. Nobody holds him chained to the demons' territory. All his suffering appears to be self-inflicted.