front that it is continually pushing further west

Ukraine is taking back terrain currently:

https://www.reuters.com/world/ukraine-has-retaken-745-sq-km-...

https://www.kyivpost.com/post/85252

Russia is successfully fighting the entirety of NATO

It is not. The primary reason that Russia can even keep up is that it has many more citizens that can be thrown in the meat grinder and can work in arms factories and it has a larger industrial base than Ukraine. If you would tell most experts before the invasion, that it would've ended up as it did, they would have laughed their heads off. They would have told you that the Ukraine army would not be a match for the Russian army and they would have told you that the response of the West would have been divided.

Russia has ~146M citizens, Ukraine has ~29M citizens within land that it currently controls. Russia's estimated GDP is $2.66 trillion, Ukraine's GDP is $225 billion. That's 5x as many citizens and a 10x larger economy.

A real conventional war against NATO would be a completely different story. Sure, NATO countries would have to catch up with drone production and defense, but a lot of knowledge transfer and co-development is already happening. But NATO's economy and population is several times larger than Russia's. If NATO countries switched to a war economy, it would not be a fair game.

Of course, Putin knows this and he also knows the real weaknesses: there is a change that not all countries will uphold Article 5 and coordination might end up in a mess if the US would bow out, because command & control has always largely been handled by the US. This is why he is constantly testing how strong Article 4 and 5 are on an escalating ladder. The other weakness are disinformation campaigns through social media, which are working really wel for them, both in the US and in various European countries.

(I don't think it's useful to talk about nuclear war, it will end up badly for everyone in a population center. There will be no winners, only losers.)