I don't know what the hell Putin is playing at. If stopping border nations from joining NATO is his primary objective, then he's doing a very good job of doing the exact opposite. Finland and Sweden are talking about joining NATO, so what does he do? Send warplanes into Swedish airspace. He's basically telling them that being neutral is no protection from Russia.

Furthermore, he's doing an excellent job of convincing people in the west who are critical of NATO that actually there is a need for such an alliance.

I'm not sure about tactical nukes. This will surely bring NATO into the conflict. Is Putin really going to escalate the conflict that much? He surely has conventional weapons at his disposal that can cause untold damage to Ukraine.

btw, why are claims of Ukraine building a dirty bomb "ridiculous"? I would agree it's unlikely, but I'm in no place to dismiss the idea outright. Ukraine are at war with a much more powerful enemy. They should be considering every possible option they have. Not that a dirty bomb should ever be an option on the table, but when tactical nukes are seemingly on the table, all bets are off.

If Ukraine are seriously worried about a nuclear escalation then it's reasonable to assume they would take extreme measures to defend themselves. Just one dirty bomb is a deterrent to Putin, and probably enough to stop him using nukes. So there's motivation for Ukraine to acquire one.