I kinda wish the author did more insane plans, cause I can think of good plans that are somewhat convincing and obvious for readers. Like, why not fake an assasination/suicide attempt in front of everyone by preplanning being hit that's nonlethal, and to whoever gives him help first become the king saying "This kindness is worthy of a king! My body is weaker cause I'm wounded, so I think this sibling should rule, father!" and then casually hand a note to the other candidates with "Brother, don't let them escape (don't show father this)" and another note to the other with "Help me fake my death, and we'll win guaranteed" Saying "Brother" for the first one, cause of the 2 brothers and have them be more trusting of the plan, and the other note for having the smarter siblings to help fake the death, already knowing not to show the note. And if the fake death doesn't work, then he just has to say he's anemic, unfit to be king. If the plan fails, then he'll just have to say his brain is slower because of the blood loss.
Yeah, the plan's entirely luck based, but it's cool, interesting, and funny, and has more potential for jokes if it fails. And I'm kinda disappointed with the manga so far, on how the plans and thought process are just simple.