People talk about NK artillery, but they don't understand how much better and more accurate Western systems are. If people believe NK can flatten Seoul, they better believe the Finns can wreck most of the Russian border crossings within a day. Most folks don't understand how close Finland is to Russian population centers as well, while Finland's population centers are better protected from land invasion.

A sea route will never be viable for Russia, as their Baltic fleet capability is vastly overrated. They have little to no amphibious capability by modern standards. They have limited missile cruisers which would be utterly annihilated within the first 2-3 days.

If it turned into a conventional warhead ballistic missile fight, the entirety of Europe will invade Russia within the week.