Russia is very bad at diplomacy and intelligence operations - as we've seen in Syria recently. First they got played at Astana and later they failed to see the offensive coming. While also failing to understand just how badly in shape Assad's army was. Russia also got played during the Minsk accords.Just neutralize the military threat and hostile government and put a friendly regime in place to filter the population, so, in the future, Russia will be able to annex the territory if it is beneficial.
Moreover, let's not forget that Russia pumped 100-200 billion USD into the Ukraine in the form of massively subsidised gas and what thanks did they get for it? Maidan. There's very little reason to think Russia would be able to install a friendly regime and keep it that way even if they get a favorable outcome at the end of the war.
Honestly, the only realistic option for Russia would be to neuter Ukraine as to make it impossible to invest in and essentially meaningless to invite into NATO or the EU. And then just wait for years as it slowly implodes on itself and just pick up the scraps.
I'm not convinced anyone would even want a rump Ukraine located in the western part of the country with Lviv as its capital. That's ground zero for crazed Ukrainian nationalism. Certainly the Poles would never let such a "Banderastan" into the EU and I doubt the Russians would want to rule over such rabble.