If they offer an invitation right now, then this is no longer a war between Ukraine and Russia, it will be a war between NATO and Russia. How do you think that ends?
WW3 may be the last world war. No one is eager to start it.
It ends with Russia losing. As long as NATO makes it explicitly clear they will not attack Russian territory nor violate Russian sovereignty Russia has no basis to escalate beyond Ukraine. A nuclear war between Russia and NATO would result in the total destruction of Russia (and everything else of course). Russia wouldn't start one unless they felt their existence was threatened.
The difference is we can use Ukraine as a proxy and don't have to send our own children to die. This war could last for 100 years and the only cost is NATO treasure, plus Ukrainian and Russian blood.
Actually there's no military difference except Russia will implode because of the news alone.
Now, until it implodes there's simply no realistic chance any nukes will fly from Russia. After it implodes, the chance is minimal, though there may be some nuclear blackmail like what North Korea does, always ending with a humanitarian shipment of grain or something.
The whole point of all this maneuvering is to preserve Russia's integrity. This is why weaponry given to Ukraine is limited in class and modernity.
This is rather cruel to Ukrainians (and Russians, because also means that NATO countries are not interested in real regime change, they are interested in controlling the current regime), but is really obvious.
I don’t disagree with most of what you said. But NATO getting fully and directly involved - As in moving in with 10s of thousands of troops to take part of Ukraine that Russia has claimed (Eg Crimea) would be a massive escalation, and I don’t think there’s a credible military or geopolitics expert who would disagree.
I don't see NATO putting boots on the ground TBF. Bombs, missile strikes, limited activity of special forces, jamming etc, - possible.
It's just too convenient to have Ukraine pay the price in lives. Ukrainian military may be getting more experience than any spectator, even a spectator with access to data from them, but it's less qualified to use that experience for improvement, while NATO militaries are very well qualified.
Also the war going on is in some sense stability, while the war ending would be destabilization in the same sense. They just prefer things moving slowly.