You are buying too much into the Russian propaganda. First, it is not NATO which expanded "into" the Baltic States and Eastern Europe. These countries voluntarily applied and their diplomats lobbied very hard to get accepted into NATO, precisely because of Russia's aggressive and militaristic policies. Why do you think that these sovereign European nations actually want to be a "buffer"? Why do you think somebody should have a right to constrain their sovereignty?
Ukraine was not even close to joining NATO, despite all declarations (moreover, Russia firmly prevented it by orchestrating the War in Donbas). With Finland joining NATO, Russia will now have 1340 km border with NATO. What Putin is going to do about that? It was never about NATO. Look at the origins of the Russian-Ukrainian War (it wasn't just 2013, the cracks started as early as 2004). Moscow always had a distorted view of the Ukrainian history and its sovereignty as an independent nation (despite signing treaties and agreements recognizing it).
> For Imperialist ambitions to make sense, Russia would have to either occupy Ukraine on a long term basis or install a puppet regime (eg like Belarus). Both are doomed from the start. The Ukraine is too large to occupy and would be too expensive. The USSR couldn't even maintain a puppet government in Afghanistan (and nor could the US obviously).
This is precisely what they tried to do with this invasion, thinking that if the Ukrainian government gets decapitated, the majority of the Ukrainian population would not resist and cooperate. Kremlin massively miscalculated because they started believing their own propaganda and their intelligence would feed the dictator with the information he wanted to hear rather than reality.