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).
Slight correction: Estonia briefly succumbed to the Mongol invaders, but Pope Innocent II rallied a crusade to liberate them. This is why it was then known as the Teutonic State.
Estonian is not Finnish but it belongs to the same language family.
Putin's actions are in keeping with a long line of Russian rulers such as Peter the Great and Catherine the Great. During the Soviet period the question sometimes was posed whether the Kremlin's actions were part of the desire to spread world Communism or just in keeping with traditional Russian expansionism.
It was Khrushchev, not Stalin, who added Crimea to Ukraine.
There is no reason to think that Putin will stop with Ukraine.