Assuming Russia invades Ukraine, what happens next, will largely depend on Germany and the rest of Europe. If the Germans are willing to shut down the Nordstream pipeline and impose strong sanctions on Russia, I don't think Russia will advance past Ukraine. However, if Europe is unwilling to force Russia to pay economic penalities for an invasion, Russia will likely quickly expand Westward.
Germany is a Russian trojan horse in NATO and they showed it by not allowing the British aid through to Ukraine. They don't care what Russia does as long as it does it outside the EU. If Germany attacked Poland it would be a different thing because Poland is in debt with the EU and a useful source of cheap labor for Germany. The EU is like a big farm where poorer countries are first pumped with cash and then, once they're unable to pay it back, farmed for installments, resources and so on. Tensions with Ukraine make things better for Germany and Poland because Ukrainians are now willing to move West and work for for cheap, but none of these countries would one day want to share a border with Russia.
I feel like Ukrainians will have to make up their minds if they want to be a part of this new Russian union along with Belarus, or join the EU. They didn't join the EU at first because they wanted to stay independent and at the same time keep relations with Russia, but as we can see this can no longer be maintained. You can't have a cake and eat it.