The Estonian border with Russia is already very defensible terrain because of rivers, lakes, marshes, and very few roads, along that entire frontier.
That defensive advantage is true for potential attacks in both directions.
Which is why the Estonian border is not the strategic threat to Russia that the Ukraine border is.
Putin’s invasion has more than doubled its border with NATO countries to about 1600 miles.
Did that open up any other points of favorable border terrain?
Perhaps, but Moscow and esp Leningrad er ah St. Petersburg are well within “air travel “ range
Also nice location on the NATO sea too