I really think Putin is a bluffer and would never nuke. I would have thought sinking the flagship of the Black Sea fleet would have brought the wrath of Putin.
Russia's "no first use" doctrine was confirmed even after the start of the war. I still don't see him nuking Ukraine, yet alone anywhere else, and I rather suspect that China has been raising objections to such action through diplomatic channels. But then, I was also pretty sure Putin wouldn't attack Ukraine in the first place, so what do I know?
This war is going to go on for years by the looks of things.
I hope you're wrong, but I fear you're right. A long war will be a disaster for Russia, of course, but Ukraine will pay a higher price. It may, too, increase the chances that Ukraine accepts a bad peace deal granting Russia meaningful gains.