Dude, you've already answered your own questions. What you're hoping for isn't gonna happen cus this is not like one of those solo levelling type of tower climbing manwhas. Please pay attention whenever they mention rankings, It's NOT about player rankings, it's KINGDOM rankings.
His power was already explained in the story. He has OP support/healing skills which (godly)boosts the already high stats of his vassals and duke(emphasis on OP cus a king's skill is unique to them), the stats they achieved via a corruption/race changing potion that MC had them take. There, that's why they're OP.
This is a unique thing that happened ONLY to MC in this game they played, that's why his kingdom is an emigma to other players, they don't fucking know why this kingdom is so OP compared to others. Like EVERY tower/gaming manwhas, this is MC's cheat, a special potion only HE discovered. Hence why MC's kingdom got legendary status among the players.
I forgot to mention that while MC before isekai was OP, when the players got isekai'd, their character levels reverted to level 1. So MC's strength is not as great as as when he was still playing the game irl. Currently, his level is still in the single digits and it isn't till very later on that MC gets a chance to farm experience by having his vassals carry him in the tower to gain exp fast.
Yah though i would say, even if MC is back to his OG level, he would still be weak physically compared to others. Since it was revealed in the novel that MC's main stat is mana/MP(which basically are the points needed for consumption when using magic) while the rest of his stats were piss poor(like hp, attack and def)
Every chapter, I wait for this fragile-like-porcelain mc to reveal some sort of jaw-dropping, wow-worthy, super duper amazing power that was the driving force of why his empire, kingdom, or whatever is striving yet every chapter I'm starting to think maybe it's bc of this piece of coding called grand duke and the unrelenting love of his subjects that smashed through the ranks and maintained its crown as supposedly the no. 1 empire or sumn.
Am I wrong to assume mc is a hidden munchkin and he's actually just some sort of compassionate king w hypersensitive wings and affectionate love?