Not really, it's right next to Murmansk and St.Petersburg. The Ukraine region is highly defensible too, they got Belarus for backup just above it, if they thought it wasn't defensible then they sure as hell wouldn't attacked it - that's the great paradox of your defensible argument. Another contradictory factor is that the land they've taken is the bit furthest away from the closest, direct line to Moscow. Your arguments make zero sense.
St. Petersburg is the 2nd largest city in Russia. The heart of Russia, i.e. Moscow is still a short throw from NE Ukraine, arguments not making sense again. You must not have heard about Stalingrad. The NATO start point could still be there because Russian holds no Ukrainian territory adjacent to Kursk. No logiic yet again, all fake arguments.
Taking a city close to the border is irrelevant, both Napoleon and Hitler learned that the hard way. You have to destroy the opposing army in order to win.
And a city is a meatgrinder anyway. Finland is gonna get half its population wiped out just trying to take and hold St. Petersburg. The Americans are not gonna attack from that direction either. The casualties from the cold winter will be worse than the war itself.
So, no, Finland isn't a serious threat to Russia.