Putin doesn't want the old Soviet Empire back. He even said so himself.Putin is ex-KGB and wants the old Soviet empire back much more than the US wants the headaches of invading Mexico.
"Anyone who doesn't miss the Soviet Union has not heart. Anyone who wants it back has no head." Or something to that effect.
He may not want the empire, but he does want the glory. The prestige.
Death by a thousand cuts.He is much more likely to succeed by taking it back little patches of land at a time over several years instead of the full-scale invasion feared by NATO. We do not have a viable defense to that strategy.
Russia has two goals. To regain glory and to keep the US in check.
Ukraine is a means to that end. But in bits and pieces.