Idk plot development BUT secret hidden skills such as high sex ability AND dual cultivation would be really great to improve MC.
He apparently can't raise his mana quantity through leveling? So I'm honestly really confused on the difference between being level 1 and 15, if the stats actually needed for combat don't change? Sounds like there is no difference.
Which is why having sex, like that one swordsman harem of a middle-aged dude, increases his stats would be great. Throw in a plot point about his level limiting the max stats earned through dual cultivation. So he still has actual need to kill monsters and level up.