Man i read this because it had an interesting premise, but it's just another generic ass revenge fantasy. I got tired of those after rooftop swordmaster.
A good way to say "Duke-sama" in English would be using a term like "your excellency," since it fits the context while not being exclusive to royalty like "your highness/majesty"
If the next chapter doesnt see MC get a massive powerup, I'm dropping this. I'm tired of seeing him get bodied every chapter despite him supposedly being a big bad plaguewyrm.