> he thinks of himself as a visionary leader, destined to make his mark on history
Well put. This seems to get glossed over. Putin doesn't have too many years left in good health and won't go quietly on gardening leave.
I agree with you and would also add that even if the NATO expansion argument is merely a facade, it's not the only one he has to play with.
OP mentioned protecting ethnic Russians in Donbas. Putin's narrative to Russians in fact goes much further than that: he portrays himself as reconquering and unifying the traditional Russian state. Let's not forget the speech he gave shortly after the invasion, in which he described Ukraine as an illegitimate state on Russian soil.
The other narrative he pushes is about neo-Nazis taking control of Ukraine. Iirc one of the aims of the "special operation" is to remove Nazis from the Ukrainian government. Which is obvious bollocks to us in the West, given that Zelensky himself is Jewish. But in Russia the war is successfully portrayed as a sort of rehash of WW2: soviets vs Nazis.