I really don't get the Greenland thing. Is there some 4D chess reason for it? The US already has military bases there and could probably have as many more as they wanted if they just asked nicely. So the whole security thing is a pretext
It does not look as if it's serious. But this is a deeply unserious administration, so it's hard to guess.
But the simpler answer is that Trump seems to personally like the idea of adding a big landmass to the US for ego reasons. He talked about it last term too with the same excuses mostly.