Nevertheless.
Ukraine is very special and there’s no proof whatsoever that Russia is interested in conquering or attacking any EU/NATO countries. And they’ve also shown that they probably couldn’t manage it even without NATO or the nuclear deterrent.
This was supposed to be just the beginning.
Russia wants NATO to be back into 1997 borders. Exactly because it wants to get EE countries back into its sphere of influence. If NATO allows this to happen (and there were people arguing for it) - Russia would blackmail these countries to become Russian vassals. And if they don't want to - it would invade them.
Russia already invaded Baltic states in 1991. Chechnya in 1994 and 1999. Georgia in 2008. Ukraine in 2014 and now.
It's not exactly a secret. Putin and his ideologues (Dugin for example) wrote about the need to "recreate Russian empire". The only reason they don't is because USA remains active in Europe and NATO remains a credible defensive alliance. But it's not guaranteed forever. There are political forces in US and the rest of NATO that consider it too costly.
It's about the gas that was found there.
Their current actions seems to have made NATO stronger ( eg. Finland and Sweden possibly joining). Don't believe Russian rethoric too much.
They don't only want Crimea. They already had Crimea before this invasion. They are occupying large parts of Donetsk and Lugansk provinces and some other parts of Ukraine and they tried to siege the capital city and nearby provinces.
It's also not just about when they talked about NATO. There were already constructing ( shell of memory serves me right) gas extraction around Crimea. It was crippled during the invasion in 2014.
It could have probably replaced ALL Russian gas imports.