Their excuses about Ukraine being Nazis and all this other crap is just propaganda bs. Russia is run by paranoid nationalists who have been trained from birth to be paranoid about invasion considering the long Russian history of being invaded. Most of western Russia is just flat ground straight to moscow.
Their ultimate goal at the outset was to secure the eastern side of the Carpathians. Ukraine was in the way. Moldova is the real prize. If they control the Black Sea coast and the eastern Carpathians then they don't have to worry about placing too many defensive resources there because the geography is just not suitable for invasion from the west there.
Then they can move more of their limited defensive resources along the border with Latvia, Lithuania, and Estonia. Ideally for them, they would take those three countries to form a land bridge to Kaliningrad and create a much narrower path for opposing forces should they choose to invade across the northern European plain. There's no doubt Putin would have already done so were it not for NATO. That's why they are so pissy about NATO encroaching so close to Russian territory.