Agreed- Whatever they can grab- Which in my view, is all of China, Korea, and Sakhalin (if the Soviet side is lucky or well-fortified from the start). The obstacles to an amphibious invasion of the home islands of Japan would be insuperable in my view.
Japan, by it's conduct in the Sino-Japanese War and alignment with European rogue states, had forfeited all western sympathy against the Soviet Union, regardless of how much the USSR was a pariah in the international system.
Plus, the western powers just really didn't want to have wars with anyone if they could help it.
Soviet attacks will be viewed in Washington, London and Paris as just desserts and appropriate comeuppance for Japan. Russian moves in East Asia would begin to alienate the west only if/when the Soviets clash with the Chinese Nationalist government, or threaten European colonial possessions. Even here, western powers will be mostly concerned with their own possessions, not each other's. IE, Soviet or later ChiCom sponsored guerrilla warfare in Indochina will mainly bother Paris, not Washington. Threats to Burma, Nepal, India would bother London, not Washington.
Now, the Soviets are not foreordained to make any moves at all. Their probable judgement will be that the juice is not worth the squeeze.
However, if they decide that the cost is reasonable, their war aims will likely center of reversing the verdict of the Russo-Japanese War, restoring South Sakhalin and influence in all Manchuria at a minimum, with a probable desire to gain dominant influence in Korea, and northern China. But, unless alienated by the Chinese Nationalists and sensing weak or non-existent local and global opposition, Moscow will likely handle it's relations with the Chinese Nationalists delicately. If they do so, the Chinese Nationalists will be unhappy at Soviet aggrandizement, but it's not their worst-case scenario because it gets Japan off their back, and a Soviet Manchuria or Korea is a buffer against renewed Japanese aggression deep into China.