I have always felt that
Russia wants to control the coastlines that Ukraine possesses. This would enable Russia to control the shipping. Also, as we have seen, the export of grains to third world countries who are highly dependent on the exports. This would have a profound affect on politics, and gives a measure of power not otherwise available. Food is a bottomline resource, something indispensible, not unlike fresh water. Those that have it are rich, and those that do not are poor.
I think that without this control of this territory, Putin would never be content to settle, no matter what. I don't see how Ukraine can agree to give up the shoreline. It's more important than who gets to speak Russian or Ukranian, and all those contrived excuses for bringing an unprovoked war. It's the matter of controlling two oceans, and everything that goes with it.