More details on the logistical issues within the Russian army: https://warontherocks.com/2021/11/feeding-the-bear-a-closer-...
Also, I would not underestimate Poland. The country spends 2.2% of its GDP on military and has been gradually increasing the expenditures further. Moreover, Poland has been making very major acquisitions of modern equipment and capabilities (something Ukraine is currently just dreaming of -- see one of my posts above) as separate budget lines. It is on the track to become one of the main military powers in the EU and is certainly a very credible power to resist Russia.
NATO as a whole might not engage in such a war, but members of EU certainly can, even without NATO.
The combined firepower of France, Germany, Italy and Spain is not something Russia can handle by itself. So the question is would Russia gets backing from China and/or Turkey to start invading portions of the EU?
That's just not true. Even if Germany decides to not help, they couldn't stop individual countries, like France or Italy from helping.
EU would have to deploy immediately at least two heavily armed divisions to be able to protect Polish-Belarusian border (which is in reality Polish-Russian border and would be first attack direction to cut supply chains from Polans) and attack Kaliningrad, what is the key in Baltic countries defense.
Nothing happens in EU without Germany agreement, so the question is: will German army going to engage? Germany is busy cutting trade deal with China and starting Nord Stream 2 which would provide Russian gas for Germany, so the answer is clear.