what's Russia's next move.

Putin and his people are not stupid, those that think he is some unhinged, delusional psycho aren't paying attention. The Russian army is still meh. Swooping in облегчение войны style and toppling a government and replacing it with a puppet is one thing, occupying a country is another, especially one that still remembers the Soviet days. So, the logical answer is, install the government and leave, telling the story of how you have saved the Ukrainian people from a corrupt government that oppressed the Russian speaking minority (which is not an entirely untrue statement). Don't forget, that's what they did in Georgia.

If anything, that kind of "save the people from the corrupt puppet of the west" and leaving a buffer where the natives are fighting each other, is a good model if your goal is to keep NATO and the EU off your borders.

The idea that they are going to invade Europe is a little far out. Russia is 1/3 the population of the US and just slightly larger than Poland and Germany combined with military tech more than a generation behind NATO tech. Unless the Chinese allied with them, that's not really an option.