Male Protagonist Noble Daughter Heroine/Harem Member Non-Weak Protagonist Japanese Not Overly Serious
isekai tensei soudouki the world of otome games is tough for mobs To Be a Power in the Shadows Rakudai Kishi no Eiyuutan (novel pdf can be downloaded)
In Another World With A Smartphone. JP, OP protag, not a weak-limp noodle beta, not overly serious (there's bits of somewhat serious and lots of SoL with some kingdom building), several princesses and nobles daughters are included in the harem.
So basically you're looking for JPN novels w/ male MCs who were engaged/married to noble's daughter(s)? Maybe something like: Chronicles of an Aristocrat Reborn in Another World – the Apostle of the Gods Who Know No Self-Restraint (the kingdom's princess & her cousin, a duke daughter. I forgot who the 3rd one parents is, but they're a bigshot in the neighbouring county & her brother is the guild master of the capital's adventurer guild) Magic Gems Gourmet The Reincarnated Boy’s Growth Log ~The Harder I Work The Stronger I Can Become!?~ (the 1st officiall fiance is the battle junkie 3rd princess) Kenja no Mago (no harem, though) Goshujin-sama to Yuku Isekai Survival! (the "main wife" is an elf county princess) My Wife is Nobunaga's Little Sister