If you are Russia, letting the US and NATO set up shop 6 minutes time of flight from Moscow in the Ukraine, is a bad idea.
If you are Russia, having the US and NATO set up shop in Ukraine with major roads leading in to Russia, major rail lines with the same gauge leading into Russia, being able to literally take out much of their nuclear deterrent capabilities in the boost phase, and having access to multiple large airfields that can support strategic bombers or lift, is a very bad idea.
In fact, having NATO and the US set up shop in a country with 233 thousand square miles or terrain that includes forests and places where you can easily hide things, and with an infrastructure that can support a large military force indefinitely, that sounds like a very bad idea if you are Russia.
On the other hand. Maybe they shouldn’t worry. We can make them a promise that we will never expand eastward with NATO, never put troops on their soil, or never build and base a missile defense system in Europe. We’re the good guys and hold our promises, so they have nothing to worry about.
You have described Russia’s casus belli. Ukraine’s casus belli is Russian troops on its soil, pounding its cities into dust.
So is Russia going to invade Finland (again)? Because they are talking about joining NATO now.
Of course after seeing Russia’s performance in Ukraine, people are losing their fear of them. They can do a lot of damage. But they can be beaten.