Honestly, this is the title I had in mind: Arknights: The Arrogant One
Also known as The Sarkaz Scholar, The Bloodstained Japanese Andromeda Flower, The Rebellious Confessor, and Medicine Can't Save the Sarkaz or Terra
No harem, no harem, no harem!
The former heroine is Theresa [with angst and sweetness]. The later heroine is the protagonist's "lowly Laterano prisoner of war (slave)."
Timeline: Terra Calendar, Year 1673 (233 years before the main story)
By the way, this book contains scenes that will delight German Empire enthusiasts (the Second Reich, not the Third Reich) and Holy Roman Empire enthusiasts. Enter with caution.
