I honestly don't see how the fear being irrational or not has anything to do with, well, anything.
My reference here is of course the Iraq war, where the neocon cabal were on a mission to remake the Middle East and the world, intending a short stop in Baghdad before they moved in on Tehran. Their stated reason was a violation of the disarmament treaty they forced on them after the last war. Now, as we all know, the WMDs were entirely fake and even if Iraq had had them that would hardly have justified a war of aggression. After all they would have been entirely defensive in nature. Yet despite the obvious futility there was an enormous effort to prove a negative. Weapon inspectors looked under every rock, half the world was contorting itself to assuage the Americans' stated concerns.
Now, what do we have in this case? Well, the WMDs, or the quest for NATO membership is real, and the peace treaty has not been implemented. Would the Russians have found another excuse, because the Kremlin is driven by an irredentionist drive to recreate the Russian empire? Obviously it's impossible to say with 100% certainty, but we didn't try at all.

So to answer the stated question: A public committment that Ukraine would not be accepted into NATO. Hardly a big ask since the only purpose of the organization is to fight a war with Russia and no one is willing to do that over Ukraine as we see now. Then a recommittment to the treaty of Minsk. Also not that big of a concession since we already accepted it after 2014.
Would that have prevented the war? I certainly don't know, but the attempt wasn't made.