No one disputes that national security is a thing. Nor that the new Axis of China/Russia and a few failed states is a global security concern.

There's "concern" and then there's "urgent" concern.
There is not a single democrat politician that views China as an urgent concern. Before somebody around here tries to say, "bbbbut Joe Biden...", please don't as all I have to do is point to the US Navy as my counterexample of why nobody on the left side of the aisle gives a rip. Or, if I'm feeling charitable, believes that the time is now to take action in any real way.

They don't.

Antagonizing European allies in order to try to get them to sell you Greenland, along with refusing to rule out the use of force against a NATO member, is probably not a beneficial tactic.

Why are you assuming that selling us Greenland is the real goal? Can we even afford it?
Why not get the Danes to make upgrades in certain areas, upgrades that are mutually beneficial in terms of trade and security?

But that would still be a dumb choice of "certain actions" to take - because undermining the reliability of the U.S. amongst our allies probably causes more damage than their increased defensive expenditures benefits our global security, relative to other options to get them to increase their defense spending.

"Undermining US reliability"? Huh? No. It's called reality. Here, let's have some numbers help us out.
statista.com - US troops europe

As late as 1989 we had north of 300,000 US troops stationed in Europe. Now it's more like ~63,000.

Do we have the ability today to ferry 300,000 troops plus their equipment someplace on the globe? Not easily, and not nearly what we used to have.

The reality is that if Shit Kicks Off someplace, the US has a vastly limited ability to project power for a sustained period of time. Therefore, our security partners need to be much more involved than we have allowed them to be for the past 30 years.