Japan already has significant ethnic Korean and ethnic Chinese minorities. And Japanese culture has absorbed a great many influences from both Korea and China over the last 1000 years, so I doubt some more Korean or Chinese immigrants would make any great long-term difference to Japan's culture. It would just be more of what has already been.
How many individuals from those ethnic backgrounds might actually want to emigrate to Japan, I don't know.
In 2021-2022, China was the number two source of immigrants to Australia, after India. If Australia is more successful at attracting Chinese immigrants than Japan, the problem isn't availability of Chinese people wishing to immigrate, the problem is Japan's attractiveness as an immigration destination for Chinese people–and surely that's something Japan can improve if they try.
Also, there a lot of ethnic Chinese people in Australia who are actually from Malaysia or Singapore; Malaysian Chinese want to escape the discrimination of Malaysia's Bumiputera policy; many Singaporeans want an escape from Singapore's cramped living conditions (small apartments). So ethnic Chinese immigrants can be sources from places other than China proper.
Currently, China is the number two immigration source country for Australia, so a declining population in China doesn't prevent it from being a source of immigrants for other countries.
South Korea and Japan do have difficult relations, and they have been getting worse. But I remember visiting South Korea some years ago and talking to some of my South Korean colleagues about how they felt about Japan, and what I noticed was wide differences in opinion – some were still very angry at Japan for things it had done long before they were born, others were critical of their fellow Koreans' anti-Japanese sentiments. I suspect some of this is political – historically, many Koreans collaborated with Japan, among others Park Chung Hee, who served in the Imperial Japanese Army during World War II, and then became South Korea's military dictator in the 1960s and 1970s. Some right-wing South Koreans still have some sympathy for the 1960s/1970s military dictatorship, and hence tend to view collaborators with Japan more sympathetically; more left-leaning South Koreans are highly critical of both.
In any event, there are a lot of things which Japan could say to try to assuage South Korean feelings on these topics. Japan's problem is that saying those things would upset Japan's own nationalist bloc. But if Japan really wants to attract Korean immigrants, maybe upsetting their own nationalists is the price of that.
How many individuals from those ethnic backgrounds might actually want to emigrate to Japan, I don't know.