Obviously (as in very obviously) this wouldn't be the big one in itself, but an entirely possible provocation that could shove things in that direction.
China is a significant inhibiting factor, but one factor among many. "Client state" is a caricature, the relationship is more complex than that.
Also, no one is "pushing Russia's back against the wall" in Ukraine, there are no M1A2s speeding to Moscow
It doesn't need to be tanks speeding to Moscow. A major shift in the ground situation Ukraine's favor (effectively signaling an eventual Russian defeat) could do the trick.
Also no one said this was presently happening, so your use of the present tense is puzzling here.
Putin is perfectly rational.
He isn't simply "irrational", but he definitely isn't "perfectly rational" either. Otherwise, he wouldn't have bullied his own advisors to a point where they were afraid to express their own opinions (or even provide rational assessments) in regard the neat little plan he decided to roll into action in the Fall of '21. And what he apparently does in all seriousness believe about his own manifest destiny (as the restorer of the Motherland's natural position of greatness) definitely isn't rational.
There's also the matter of his successors, who won't be nearly as smart, and will likely be far less rational.
The empty threats are working (as evidenced by this thread),
I guess everyone who doesn't agree with your assessments is either paralyzed by irrational fears, and/or a useful idiot, or just looking for a cop-out.