Added bonus(but if not it’s fine) a ruthless mc who is not a simp and prefer no crazy nationalism. But again beggars can’t be choosers so give me whatcha got
The World Online God and Devil World I am the Monarch Dimensional Sovereign <-- this one isn't much for kingdom building, but he does have his own base it's just not the main objective. Stronghold In The Apocalypse <-- I just picked this one up it's pretty good so far around ch 50 so far.
12 Kingdoms. Not joking, read it. Look at the ratings. Even more lopsided than the leaning tower of Pisa.
I read a lot of this kind of book, and properly ruthless protagonists don't make for very good kingdom building novels. Usually it just leads to them being obnoxiously selfish and the plots tend to get pretty silly. For example, I was reading "Iron Dynasty" and it had some decent bits, but then it got into stupid arguments akin to "European colonialism did horrible things to people, so we should just copy them!" It's also going to run counter to your ultranationalism thing because that's what extreme ruthlessness is always going to lead to. I could recommend books that do a much better job of this but none of them are translated. I'd be happy to list titles if you're still interested though. It's a great book but it's way more world building than kingdom building. It's too bad that they don't make isekai like this any more.