Estonia was deliberately repopulated with Russians after WW2 and narrowly voted for independence after the collapse of the Soviet Union. There are now more ethnic Russians than Estonians in that country, so a Russian annexation would not be entirely unpopular. Estonia is a NATO member, but "internal unrest" (a la Ukraine?) would not necessarily trigger a military response by other NATO members. Also, it is a remote country for which supplies and support would be difficult. Are they next in line, or will Putin put on hold his dream of restoring the Russian Empire?
I've been there jwoodie. It isn't that remote. I've had breakfast in Vilnius, which is in the southern part of Lithuania, and driven in a car to Tallin, which is the northernmost part of Estonia in time for dinner. About a 10 hour drive when you include a lunch stop in Riga.
You might be off on you population estimate also. There are more there than in Lithuania, but in Lithuania they have only a very small population there. I think they have more Polish than Russian. Latvia has more of a Russian population than Estonia, but only slightly. Latvia and Estonia both have maybe 25% Russian populations.