Let me put one thing first, I really could not care less who is to blame for what, whether Russia is the bad guy or the US or NATO or the EU or the Illuminati. I am interested in understanding what went wrong and maybe get some insight into what should be done better the next time.
With that out of the way, after the invasion started I got somewhat interested in the conflict and how it developed. Pretty quickly it seemed pretty clear to me that NATO expansion was most likely the main driving factor behind Putin's decision to start this war. But for many in the West it seems incomprehensible that NATO's actions might have played any role, even less that Putin's reaction might to some extend be understandable.
So following the invasion this narrative spread in the West that Putin is simply mad and wants to rebuild the Soviet empire - first Ukraine, then the Baltic states, then the rest of the world. And as I said, I could never find much evidence for this explanation. The thread started with the following.
So when putin claims casus belli over another nation's land due to previous ownership a whole bunch of times [...]
So I thought casus belli should maybe be best addressed in Putin's address after the invasion started and I never read it completely before. In there essentially nothing about territorial claims. So I asked for something else as I was of course aware of that other explanation. You pointed me towards On the Historical Unity of Russians and Ukrainians which, as you point out, talks about how Russia may have legitimate claims for territories in former Soviet states.
My point is just that the language in there does not match - at least in my opinion - what some people want to explain with this. »All other territorial acquisitions are subject to discussion, negotiations, given that the ground has been revoked.« I just can not see the imperial Russia in that language that would make this a reasonable explanation for the invasion, especially if you contrast it with the clear and direct language used to address NATO expansion and similar topics, repeatedly again and again for three decades now.
So if you think I moved the goal posts, then I probably just did not make clear enough where they are. I am looking for evidence that supports the theory that mad Putin simply wants to rebuild an empire as his legacy. And not some bits here and there, but something that makes it similarly plausible as NATO expansion as the leading cause, which is supported by countless speeches and documents with no uncertain language spanning decades.