“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? “
No.
That’s the amazing part.
The border with Norway is extremely remote Arctic with very few roads.
the border with Finland excluding the Vyborg peninsula is heavily forested with very few roads (that’s why the Ussr had such trouble in the Winter War). The Vyborg peninsula north of St Petersburg has large numbers of lakes, rivers and marches.
The Estonian border has rivers, lakes and marshes with very few roads.
The southern part of the Latvia border is vulnerable and was the invasion route of Army Group North in Barbarossa.
The Belarus border north of the Pripets Marshes is vulnerable and was the route for Army Group Center in Barbarossa.
The most vulnerable border, and by far the longest stretch is the Ukraine border between the Belarus border/Pripets marshes and Karkhiv. That was the Route for Barbarossa Army Group South.
Also, the 1991 border South of Kharkiv is very vulnerable to a march on Moscow.
For anyone who is interested, this is a Canadian Reserve Staff officer going thru a terrain analysis of the Ukraine Russia border.
(relevant section starts at the prompt)
https://youtu.be/00RGPa9CQgw?t=915
Finland will be happy to learn that.