Ukraine can be burning on your front door, and you still think EU is safe. Guess what, EU is safe only because of NATO. Russia has attacked basically every neighboring country in Europe that is not in NATO.
Nukes don't work against salami slicing. Russia takes Estonia tomorrow, are you going to throw a nuke against Russia? No, you'll won't risk annihilation for Estonia, so you'll ignore it. So the next day Russia takes Latvia. And so on so forth. The only way of fighting salami slicing is applying proportional response, which requires a conventional army.
Also, Ukraine living standards isn't very high right now, regardless what their PPP is, and getting conquered by Russia, means you get used as battle-slaved for the next conquest. This snowball tactic was innovated by the Mongols, using the conquered people as cannon fodder for the next war.