Exactly how extensive is the character creator in this one compared to Kingmaker, and is it possible for modders to further improve and/or modify the character creator for players to be able to create very detailed characters that can resemble their character's portrait as accurately as possible?

I suggest for you to look this video:https://www.youtube.com/watch?v=OtaPUXp7sn0

There is option to import you own pictures from well.. ..what ever you want to. So I am 100% sure that there will NEVER be (even with mods) to put enough details for every single possible picture you want to take and use your PC picture.

But there are quite of plenty of option for color and so on. Naturally things like "I have custom picture where I have half-orc with 1 broken tooth and another one is ivory yellow, orange mohawk, tattoo of red dragon in his back, bluck fur in chest and "HATE" + "LOVE" tattoo for his nuckles, he have dollar picture in his eyes and his skin color in purplish yellow. He is wearing weapon X + Y, have sabretooth tiger fur around his neck and so on" are next to impossible to made seen in the game. Not to mention if you choose to use some anime portrait or so.

Might as well just play as a half naked barbarian in appearance as literally any class instead. I'm a very technical and literalist person, if I can't make ultra-detailed characters then what's the point? Imagination? It's not enough.

Well, every single potion, weapon and so on are shown in your PC and companion. Same as different armour, shield, robes and other things. Even little details as a torch hanging in your belt is to be seen...

Outside of a few UI changes it seems to be exactly the same as in Kingmaker going by the video linked by Count-JN.

Actually was expecting more this time around as the first game's character creator was fairly dated... will see what it's lkike on the second.