I think the primary reason Putin wants Ukraine is that he sees a difficult future for the Russian economy over the next 10-20 years in the declining importance of fossil fuels (lest we forget the oil price war in 2020) and thinks that expansionist nationalism is an easier way to secure his government's stability against its own people than trying to reduce his country's dependence on energy exports.
The actual security of Russia re: NATO is not really in question here, though you're right that he may not believe this, and we can't be sure that he does. However given the lukewarm international response to his annexation of Crimea, I'm inclined to believe the economic explanation more. He has just decided (probably correctly) that Ukraine is not a country that NATO will go to war to save.
The actual security of Russia re: NATO is not really in question here, though you're right that he may not believe this, and we can't be sure that he does. However given the lukewarm international response to his annexation of Crimea, I'm inclined to believe the economic explanation more. He has just decided (probably correctly) that Ukraine is not a country that NATO will go to war to save.