North Korea has a massive army. But their technology is inferior to most of the countries they taunt. Most of their machinery is old,and in a poor state of maintenance.
That’s not to say they are toothless.
Their biggest problem is that they don’t have the resources to carry on an extended conflict. They would run out of things like fuel for their tanks in pretty short order. Most analyses by military experts has indicated that they could invade some small islands, or make potentially some small advances, but any conventional war would be unsustainable. Japan is military hamstrung by the treaties signed after WW2. These agreements prohibited offensive weapons build ups,but did not include defensive capabilities. Japan is quite capable of defending themselves from NK. (China is a different story.)
Their nuclear weapons program has to be taken seriously,as they will eventually figure out the technology to deliver a warhead.
China doesn’t want regime change in NK. It would create a huge leadership vacuum in a country that already has widespread famine, infrastructure issues, and a generally unhealthy, and brainwashed population. The subsequent humanitarian crisis, and refugees pouring into China and South Korea would be a disaster. It would further destabilize that region, and all ramifications would likely be negative.
Unfortunately , there is no current “good” options on the table for dealing with NK.
They have a highly volatile leadership, which struggles with cooperation, and paranoia. The real sad thing is the treatment of their population. I feel sorry for them. I hope for some positive outcome for them.
Ultimately, the North Korean people themselves may have to be the answer. They need to collectively rise against, and overthrow Kim. But it would be a monumental task, and Kim could easily use kill hundreds of thousands of his own people as he goes down.