Slave Harem never had smart characters in it, is a healing story with little mind put into it, the same routine repeating for tens of chapters is normal, I'm talking from the Web Novel experience, I believe the Light Novel changed things as it would be hard to keep paying customers with the original plot.
MC never was much bright, only had a few moments, he is so dumb that a skill that have everything worte in the title confuses him.
Abstinance Strike is exactly what it says on the can, don't fuck your harem and you will critical attack, of course is useless to him.
The manga is better than the WN because it doesn't repeat stuff, it just passes from one thing to other when explaining, and have a good art style.