I have to say I have yet to hear a theory about Putin's motives that is satisfactory. That he wants to bring back the USSR is not entirely clear to me, especially since the heads of the USSR were probably not in the most enviable position compared to Putin right now. Whatever his other traits, I've always observed Putin to be concise and clear in his reasoning when asked questions, and at least my impression was that he was a mafioso quite in control of his game. Why he should do something this risky, where the gains are hard to see and the trouble potentially far more than any gains he'll find, is a mystery as far as I'm concerned. Why do this? As a mafioso, going to war is something you'd do when either backed up against a wall, or else when you know you'll get a big payoff. What's the big payoff? Or is his back up against a wall for reasons I'm not aware of? The whole thing is weird. I don't particularly accept that he's just delusional and is making attacks in the outrageous assumption he'll magically take over eastern Europe and be a mighty king.
All wars begin often with miscalculations. There are plenty to go around here.
It seems partially obvious that Pooter miscalculated a number of things here.
1. The ability of the Russian army and air force as compared to his operations in George and Crimea.
2. The resistance capable by the Ukrainians.
3. The economic response by NATO.
4. The PR/information ability of TikTok/Twitter to win the information/propaganda fight
5. The leadership of the Ukrainian government
I think that Pooter expected Ukraine to fall within 2-3 days. I suspect he believed he could move so fast that NATO and the EU would accept the invasion as a fait accomplit and that he would only have to suffer economically for a short while.
Some things Pooter did get right
1. NATO would not become involved due to fear of nuclear reprisal
2. That Europe would not shut off the gas.
The war isn't over yet. Pooter can still win. Ukraine can still fall and he can threaten Europe and selected countries can fold under the pressure of invasion or nuclear attack. So far NATO has shown that they will support a buffer country economically and logistically, but has not been shown that it will risk nuclear war if a member state is invaded.
Pooter uses military power, audacity, and threat to punch above his weight economically. The goal is to increase the power and threat of Russia. Bringing Ukraine back into the fold of Russian hegemony, like Belarus is now, increases his economic and military threat capability. Consider how much stronger Russia would be with Ukraine fighting for it instead of against it. His plan is basically to extort Europe. It's simply a gangster taking more territory to be stronger. Think Marlo Stansfield. Power and respect is what Pooter believes Russia lost by the dissolution of the USSR. It's what he wants back. Hitler was basically of the same mind when it came to "German humiliation" after WWI.
Power and respect/fear.
If you went through Russia in the 1990s you probably do have a feeling that the fall of the USSR was bad for Russia. Russia goes from being a superpower to a "sick man of Europe". The power and respect of the USSR was tied to it's military capabilities. He wants to rebuild that. He's already got Chechnyans fighting for him in Ukraine. He wants Ukrainians and Belarusians fighting for him too. He wants to walk into the room at a summit with the G7/G8 or a EU summit and be the guy everybody is afraid of and bows down to. That's it.
That's my theory/take. I don't think it's terribly complicated. I think it fits with what other world leaders have said about him.