I think the original Russian plan didn’t even involve any control beyond Donbas. Now, it is clear that NATO is seeking to use Ukraine as a proxy in an all-out war and also to release the frozen Russian assets to “rebuild” it.
Leaving Ukraine in existence is a potential of long-time instability for Russia.
Just looking at NATO’S heavy military buildup in Ukraine these past 7 years or so and all the shipments of military equipment etc. They were pumped for a war they were preparing to have - without question.