I would start with two observations:
1. Putin’s goals have turned out to be more expansive than many (though not I) expected.
2. There are increasing doubts about Putin’s rationality.
I’ll accept #1, which has been my view all along, but put aside #2 for the time being.
In my simple model, in addition to a partial restoration of the empire, Putin desires a fundamental disruption to the EU and NATO. And much of Ukraine is not worth his ruling. As things currently stand, splitting Ukraine and taking the eastern half, while terrible for Ukraine (and for most of Russia as well), would not disrupt the EU and NATO. So when Putin is done doing that, he will attack and take a slice of territory to the north. It could be eastern Estonia, or it could relate to the Suwalki corridor, but in any case the act will be a larger challenge to the West because of explicit treaty commitments. Then he will see if we are willing to fight a war to get it back.
There are fixed costs to mobilization and incurring potential public wrath over the war, so as a leader you might as well “get the most out of it.” Our best hope is that the current Russian operations in Ukraine go sufficiently poorly that it does not come to this.
Addendum: And some good questions from Rob Lee.