They want all of Ukraine, but in the reality as you have seen, you can't so what do you try to do? Hit the capital and hope Kiev surrenders. Got yourself a large country added without much cost. Why the hell would you believe they won't annex after just recognizing Luhansk and Donbass which will join Russia later because they don't want to look like invaders and take em by force but make it look like the locals got independence and will ask to join Russia afterwards, not to mention freakining invading Ukraine immediately after recognizing independence for those two regions now prior to preparing invasion force the last few months. And let's not get started with Green men invading Crimea and annexing that as well couple of years ago.