The route through Belarus was what I was thinking.
Good of you to post the map for evreryone’s attention.
But thinking on it more, Russia still has the bulk of the airlift capability in had in the 1968 Czech invasion. With the short distances they could be on the ground before NATO understood their target and intentions.
Once again it would come down to the will and resolve of the UK and France just as before.
Once the Baltics are occupied the NATO options narrow down to what? Air war against Russian SAMs? Land war jump off from Poland?
They would probably beef up Polands defenses but wave goodbye to the Baltics.