> His plan isn't to take entire control of Ukraine militarily but to swap the legit government with a puppet one he would control at will. Once he succeed, which is a matter of a few days,he'll gradually withdraw most of the forces, things will slowly return to normal and in a few years Ukraine will essentially (if not effectively) annex itself to Russia

I wouldn't be so certain. I don't think Ukrainians would just comply, many of them would fight against such a thing and would know the new regime is fake. Furthermore, Kyiv is a big city with peculiar geography. Urban fighting is hell, and if Ukraine decides to make a principled stand there it could take weeks of bloody fighting before it falls; and if Ukraine's government evacuates to Lviv in time, and continues the fight from there, it might result in a long struggle, regardless of who gets installed in Kyiv by Putin.

Oh and we don't know how the Russian public will react if the war gets to an urban bloodbath going for weeks or months.