Yeah, interesting essay.
If Putin does decide to invade Lithuania, Latvia or Estonia, then these NATO member states will rightly defend themselves, aided by other NATO countries.
I have already heard about Putin's essay, which the author links to. Putin does indeed argue that Ukrainians, Belorussians and Russians are the same people. Although it's perhaps important to note that he doesn't include ethnic Lithuanians, Latvians or Estonians in his analysis.
I'm not 100% on whether Putin will attack the Balkans. If he does then he can have no qualms when the UK and US get properly involved.
Personally, I feel that if Putin has a next target it will be Moldova. Transnistria is an eastern area in Moldova which is populated by ethnic Russians who are looking to Moscow.
There's one thing I wonder about the West's wonderful relationship with Ukraine and it's this:
What does the West get out of it?
I understand what Ukraine gets out of it - lots of free military hardware costing billions of pounds/dollars.
Does Zalenskyy intend to pay us back at some point?
Let's imagine that in 5 years time, Argentina invades the Falklands. Would Ukraine give us some military hardware and equipment, or help us in some other way?
The West-Ukraine relationship seems to be one-way. Relationships without reciprocity rarely last long.