NATO or not, I have my doubts here. Does anyone believe any sane US president would risk a destruction of the USA to revenge / protect Sweden?

It's of course unclear what would happen, but I don't think Russian strategy can rely on US not responding.

Equivalent retaliation would be targeting a city of some of the Russian allies, but it's not possible since Russia doesn't have allies ;)

If the decision makers think "we can use as many nukes as we want because we have a huge stockpile and can level anyone who even tries to respond in kind", we're already on borrowed time.

I think it is in Russian elites’ and Putin’s self interest to continue the status quo and grind down the West in Ukraine, plunder it, and leave the long-term consequences to Russia in someone else’s hands. I’m doubtful they’ll actually use atomics unless the walls are violently closing in. Even China backed way off when Putin loosely waved around nuclear rhetoric in the opening months of the war, and asked in nervous diplomatic speak, “chill, broseph”.