The question, after Putin takes as much of Ukraine as he thinks he can handle, is who's next? Estonia has a Russian-speaking minority. But they are also a member of NATO. Will Putin hold back for that reason, or think that this is his chance to show that NATO is a paper tiger?
Comparing Crimea and Estonia is absurd.
Crimea has always been Russia, as far back as the 12th century, albeit with Mongol/Golden Horde interruptions. Stalin only put it in control of Ukraine to increase the Russian population within Ukraine. They agreed to stay with Ukraine after the collapse of the Soviet Union only because they were given a unique status as an autonomous country within Ukraine, and when Ukraine nullified its election results and moved to erase Crimea from the map, they joined Russia.
Estonia only became part of the Soviet Union during World War Two. Apart from occupations, such as Finland was also subject to, they’ve never been Russian. They’re not even Slavs; they’re Finns. Estonia (the Teutonic State) never even succumbed to the Mongol empire that subsumed Russia. Yes, after Stalin conquered Estonia, he moved Russians in to occupy, but as of the 1930s, there were only about 92,000 Russians. By 1989, there were 470,000, but a quarter of THOSE returned to Russia.
The Ukraine has historically been split between Russia/The Golden Horde and Lithuania/Poland. The scary thing is that Crimea was very easy to snap off of Ukraine; Luhansk and Donestk are very Russian, but also had been very integrated into Ukraine; About HALF of the rest of Ukraine is culturally Russian, but much more “Ukrainized” (if I can coin a term).