It is not Russia's interest to occupy Ukraine, imo. Ukraine is a massive country with tens of millions of people, many of which do not want this invasion. Russia's best interest is to pressure Ukraine to surrender and fulfill their demands. The more territory and important places Russia takes, the more pressure that puts on Ukraine. Occupation is the worst case scenario, but I hope it won't come to that.

Installing a puppet government isn't the only solution. I think Russia's best chance is to weaken Ukraine's army and its military infrastructure, and then enforce a federalist rule in Ukraine. This will give more power to the pro-Russian parts of Ukraine, and make it more difficult for Ukraine to be one big United country against Russia.