Stalin turns east after Germany's defeat

Would Stalin try to take the rest of Europe if the Western Allies still had to invade Japan?

Probably not; he would have reasons to join operations against Japan and might pursue greater gains in China instead.

Stalin is unlikely to exploit an invasion of Japan by attacking the rest of Europe. Soviet participation in the campaign against Japan would offer its own opportunities. He might instead try to secure much more of China. Europe had comparatively little left to offer beyond territory and population, while further expansion would provoke the United States. The scenario therefore shifts Soviet attention east rather than assuming unlimited westward advance.