It’s been my opinion for a long time that Japan has everything all ready to go and likely could assemble a nuke within a month if they wanted/needed to.
Technology and know how aren’t an issue, equipment like centrifuges wouldn’t be an issue, they have plenty of nuclear plants to provide the fissionable materials.
They have the lift capability from their JAXA satellite program as well.
I agree; they would be dangerously near-sighted if they did not account for a change in strategic alignments. They have taken the position of satrapy to the U.S. on defense since their crushing defeat in WWII. I cannot say that I would be completely comfortable with a nuclear Japan, but I am not anxious to learn to speak Chinese at my age either.