Well, he doesn't want to take over the world, he wants revenge on his previous "Friends/Team". If he just takes over, they might run away. He also wants to know what this "Master" business is. Everything needs info, and you can't simply extract info by force, you get a lot of misinformation.
Also, there's the business of the ones pulling the strings behind the kingdoms.
What are you talking, of course he can extract info by force. Did you forget thats what they have literally been doing with people they have captured. Full on mind rape. And run where? Where the hell are they going to hide? What can they possibly do to avoid this guys endless power of limitless cards and infinite summons?
And he does want to take over the world, thats what he's literally doing. He's one by one enforcing his will on all the other kingdoms and making them do what he says or else. He just justifying it by saying he's helping humanity. End of the day though, whether right or wrong, agree or disagree, etc. etc. Thats what he's definitively doing.
And, he can just crush them as he's done before, mind rape them as he's done before, and get this over with. Just take over the dam world and they'll have nowhere to hide.