I don't think we can say that the nuclear umbrella is illusory until there is direct conflict and invasion across the frontier between what's left of NATO and the Warsaw Pact. Ukraine's incursion into Kursk does not realistically threaten the long-term integrity and security of Russia, while Ukraine's self-defense has demonstrated that the cold-war fear of Russia blitzkrieging its way to the Atlantic is not realistic.

Maybe Putin's non-invocation (up to now) of the nuclear umbrella shows he is as eager as anyone to maintain it as the ultimate protection against existential threats. The US's sharp turn away from globalism is a gift to Putin, and, given that the US population is strongly divided over the changes the current administration is making, that might be threatened if Russia were to act more belligerently than it already is.

Personally, I'm more concerned over what will happen if and when the situation becomes one in which Trump realizes he's going to be seen as the president who 'lost' Taiwan.