“...is allowing Ukraine to live as free and independent nation an existential threat to Russia?”
No country STUFFED with NATO Bases, CIA Bases, and Secret Biolabs, all run by foreign entities can be free and independent. It was either the Neocons running Ukraine, or the Russians, and Russia didn’t like the first option.
Again, Zelensky had already agreed to a neutral Ukraine and no NATO membership. Nor was the present Ukrainian government “installed” by the US. But acknowledging that would destroy your narrative.