I've finally done it. I reached level 20 with Charname and also X in all kingdom stats. Here's how it went:
I started a new run as a Sylvan Sorcerer with a pet Leopard. Shared experience is turned off, only party members get experience. And only skill users get experience from skill checks, not the entire party. Speaking of the party, it was a three person team for the most part: Charname, Valerie and Amiri. I only accepted other NPCs temporarily, to complete their personal quests. When the Pitax chapter started, Charname was level 18 or 19. After succeeding at several skill checks in the tavern at Pitax, she reached level 20. Valerie and Amiri were level 17 I think. I also bought the Ring of Circumstances at the Rushlight Tournament, and a few other items.
As for the kingdom stats, I reached X in all ranks after the Pitax chapter, during An Ancient Curse Part Seven. Mind you, I didn't explore the entire map, nor did I claim every region, and I only upgraded one region so far. I have 199 days remaining until the Ancient Curse is upon the kingdom. I don't know if that will be enough for exploring, claiming the rest of the regions and upgrading them. But I did research all of the curses, completed all of the economy projects, and trained all of the advisors that can be trained.
Another thing that I want to mention is that since Charname is level 20, I can hire some level 20 mercs instead of trying to level up Valerie and Amiri. They're expensive though, apparently a level 20 merc costs 200.000 gold. But if you have the cash (which I don't, I spent it all on shiny items and kingdom stuff) you can have a full party of level 20 NPCs before going to the House at the Edge of Time.
As for the artisan's masterpieces, I only got some of them. I think they were: Onslaught, Grandmaster's Rod, Ambrosial Attire, and Mastery. I didn't get Bokken's potion, but maybe he'll craft it before An Ancient Curse Part Seven ends.
Honestly, this game has a lot of replay value. I've been meaning to start Wrath of the Righteous but I'm still playing Kingmaker for some reason, lol.