I think you might be missing the natural resources in the eastern part pf Ukraine (newly discovered oil field, also lithium deposits). Putin's all about exploiting natural resources.

Also, if you look at this with the game of 'go' in mind, it makes sense to at least try to capture a land bridge between the RF and Crimea. As I understand it the first countermove after the Russians seized Crimea was to turn off the water (literally a dam across a river; ag land in Crimea is undergoing salinization from the lack of irrigation).

Not that I wish the RF luck in this endeavor - but it at least seems rational.