Some pundits are in agreement with perhaps the endgame is not to take the entire country. A theory is that he wants Donbas, I'm not sure why. He also wants some swath over on the Western border abutting Poland, Slovakia, Hungary & Romania as I think a buffer. And finally, perhaps most interesting, a big southern swath just above Crimea.
Why the third area is interesting is because of water, the salty kind and the drinkable kind. Crimea apparently is a barren wasteland. It got like 85% of its water from a canal off of the Dneiper river in southern Ukraine. After Russia annexed Crimea in 2014, Ukraine cemented up the canal - No more Ukranian potable water to Crimea. With the annexation of Crimea, Russia also got all of the water rights off the coast in the Black Sea where there are a bunch of oil reserves and existing platforms and such.
Lastly, I was reading an opinion piece in the WSJ written by a Russian historian. He was saying Putin may not want the whole shootin' match because then he has to take on all of its ills, not to mention millions of very angry anti-Moscow people. He cited Hungary in 1956. There was a big uprising against the USSR, Russia had to roll in with tanks and then take total control of the nation. With it, came the responsibility of propping up the place, maintaining order, all that nation building stuff. Very expensive and resource heavy.
And maybe to do all that, Putin has smash Kyiv, take what he wants and leaves the rest behind.