I find the rationale for Russia's aggression fuzzy at best, but I think to view this as a Russia versus NATO or Russia versus Ukraine is, perhaps, less helpful than it first appears.
In short, I suspect this may be Putin's way of applying pressure to Russian elites. The actions proceeding from him appear desperate (i.e. the brazen assassination attempt of Nalvany) and suggest that Putin feels much less secure than his strongman portrayal suggests.
This war achieves something Putin lacks hegemony over, restricting the lifestyles and wealth of Russia's nomadic elite (and especially their assets). Elite members of Russian society will almost surely be targets of Western sanctions. This may secure Putin and his cronies and ensure a desirable transition of power by kneecapping potential contenders of the Russian throne (for lack of a better word). No doubt western sanctions will be leaky and allow some elites through relatively unscathed, but it may restrict their latitude of choice sufficiently.
I think this is Putin versus the elites. Elites who may be feeling comfortable usurping Putin and installing someone pliant to their interests. Putin may be simply reminding them of what he is capable of.
I suspect Ukraine was selected due to the presence of a large minority of citizens neutral or proponents of Russian rule. Other former Soviet states seem much more reluctant to be Russian subjects. I think Russia is simply conquering territory of peoples who will not oppose its rule.
Perhaps Putin is insane, or delusional, but evaluating public actions without knowing what went on behind closed doors feels too rash.