The man has proved he's a total idiot. He invades Ukraine basically because he fears Nato expansion East,what does he get, Finland and Sweden joining up to make the border he needs to defend thousands of miles longer. He's a loose cannon who holds the codes to Russia's nuclear arsenal. If you don't think he has the balls to use it well we'll have to agree to disagree on that one. Like Hitler did he has surrounded himself with acolytes who will gladly give their lives for him, Medvedev is if anything crazier than he is,don't expect an in house assassination before he goes for total world destruction.
Sorry for repeating myself, but I would really question that statement - I know its what they have said repeatedly, but that does not mean it is true.
What he probably fears is not NATO expansion (I mean, Ukraine was very unlikely to be invited pre-2014 and never was after it), but EU expansion. He has watched what has happened to Poland, the Baltics and most of Eastern Europe since admission, and more importantly he knows his population has watched it too.
If Ukraine had gone into the EU trade pact (the last minute rejection of which triggered this tragedy), they would almost certainly have seen a lot less obvious corruption and an increase in the standard of living for most of the population. The inevitable outcome of this would have been elements within the Russian population thinking why they couldn't have that too; indeed that still might happen if the Serbs and Turks join (which I think is more likely now in terms of geopolitics and the need to protect the EU against anything crazy happening in the US in 2024).
So what he urgently needs is to put as much between the EU and Russia as possible; both domestically (by convincing the Russian population that the EU is both an enemy and merely a satellite of the US) and internationally (by appearing such a threat to the Baltics, Poland and others that they would block any moves for the EU and Russia under his rule to get closer together).
In short, having NATO battlegroups all along the border is not that much of a problem to him, given that they will never invade and the "threat" would support a paranoid state. Having prosperous EU member states along his border is much more of a problem to him, at least whilst people in Russia think they could have a part of that too (and they absolutely could).