I don’t believe Kim has the tech to actually put a nuclear bomb on a ICBM. For one, they are very heavy. So while an empty ICBM might reach trajectories that could hit Japan, its a long way to one that can hoist a heavy bomb.
There is also the issue of re-entry and timing. He just isn’t there. The second issue it Trump should just keep silent and see what happens with sanctions. And seize and board ships, arrest those with contraban against the sanctions. That is one way to squeeze them. At the next missile launch there should be a response. First shoot down the launched missle (if we can). We don’t do that because if we were to miss it would look really bad for Japan. Second, a strike to destroy their nuclear reactor and sites known for production of materials and bombs. This strike will show how helpless they are. Unfortunately, we only have nuclear options for such a powerful strike. We should develop much larger cruise missiles that can ferry something more like our MOABs over. Our current cruise missiles are designed to be sub fired. We don’t need that. We can launch them from a carrier just fine and they can be three times as big. And why not smuggle a case of Cristal loaded with polonium or some other slow acting poison? We all know Un loves his Cristal. Anything that got into the right channels would make its way to one of his dinners.