Except that Finland was part of the Russian Empire (but not part of the USSR), so if Putin wants to reconstitute the Russian Empire of tsarist days, Finland is indeed on the menu.
What you say is true, but it’s a stretch to be afraid of this. Finland has been independent for more than a century. It was never part of the Soviet Union. It has not been a part of the current Russian state. The Tsar is gone.
A powerful nation can (I suppose) invade wherever they want and if they are strong enough, maybe they can get away with it. But in terms of any sort of “justification” Russia has absolutely no basis for invading Finland.
On the other hand, in the case of Ukraine, while strong arguments can be presented on both sides, it is well understood that in the eyes of Russia, at least, they have a claim to Ukrainian territory.
It’s apples and oranges.