If these parts of Japan joined the United States, how drastic would the change in discourse? I don't the japanese culture approves of the majority of things we were already starting to see during 2020.
Well, consider that Japanese culture expects innocent persons wrongfully arrested by the police to confess so that the police can save face by not admitting they were wrong, and in return the charges against them are quietly dropped. Consider that in Japan, it is completely normal for the police to have an omnipresent, almost Big Brother level presence throughout everything.
Even though Japan is strongly influenced by Western culture and legal philosophy, Japanese culture is radically different from American culture in ways that make them radically incompatible. While they are certainly a functional society in their own right, they're a functional society based on a lot of the opposite solutions to the same problems, differing in ways that are completely orthogonal to the American left/right political split and would honestly horrify both.