I'm not at all confident Russia could conquer Poland though even if it's left to fight alone (together with the Baltic states). It spends ~65% more on its military than Ukraine, has full access to western tech and way less internal problems (good luck finding any region in poland where > 0.x% of the population would want to join Russia). Even if Russia 'defeats' the Ukrainian military (which I don't think is at all certain) what is Putin's next step? Any regime they install in Ukraine would require enormous resources to maintain power and I don't see how can Russia afford that, at best they can bite off some regions in eastern Ukraine.

Also most Russian don't seem to be even close to being as enthusiastic as the majority of Nazi Germans were when their tanks rolled into Paris. Russians seem to be mostly apathetic, they don't really support the war but also are not willing to actually stand up and do anything about it. That might change if there is not quick victory/large amounts of civilians start being killed/sanctions actually start working.