Look up the term "Lebensraum" from WWII era. That might put this into context for you.
Eastern Ukraine is Russia's Sudetenland.
However, in terms of a "buffer zone", strategic geography; leading to a nation that's "self-sufficient in terms of resources and territory". In my view, annexation of Crimea fits that strategic idea more than to believe that it was an ideological move to regain long-lost land.
Plus, with 40% of the world's grain export, Ukraine is a fertile target for Russia.
Russia also exports food, so it doesn't really need a "breadbasket" region. Ukrainian heavy industry (which is mostly in the East!) is a more lucrative prize. But there's really no economic justification for a grab for the entirety of Ukraine. Donbas alone would be a net sink for a very long time.
There's plenty of (internal) political reasons, though. Especially if you accept that Putin really believes the tripe that he is saying about Russia and Ukraine, which I'm increasingly convinced is the case.