Depends. It's all about proportionality and the fact that Russia is in a favourable position at the time of writing as well as the fact that Ukraine itself doesn't have any nukes. I think he'd rather wait with the "red line" trump card until the future conflict with a Nato country where he challenges article 5. He'd be able to use it to save the situation if he "fails" the challenge of article 5, that is, no split in Nato is caused over unwillingness to help. If the entire alliance is resolute in a military response he can just go "If you attack any of our targets in Kaliningrad or on our territory" (In this scenario I'd imagine Nato's response would be gradually intensifying as a response to a, let's say hybrid invasion of one of the Baltic countries that's not entirely an invasion like Ukraine. That'd still get the entire alliance to come help, and his idea is to be able to make the threat real and tangible but at the same time not bad enough that the US won't feel the need to intervene due to isolationist attitudes. If the Nato response is slow but resolute, Putin can easily back down by showing a clear red line that Nato trusts is real and will thus be able to save face while deescalation the situation if the challenge fails. The equation win = Diminished Nato influence and potential for us (Russia) to be more aggressive in Eastern Europe or lose = Nuclear War and the annihilation of Russia as an entity doesn't seem all that worth it. Plus, the vaguer he is with his threats the safer Nato feels to send weapons to Ukraine and allow this because they know it's not a risk. So in short, bringing the nuclear card up is only a viable option (for now) if an actual threat is posed to Russia, nuclear threats and posturing because of western weapons causing damage on Russian soil might not be optimal in the long run because that might just get Nato to view it less as an actual threat.
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