The scenario you describe has already come to pass. Russia has NATO on their doorstep since Finland joined, Russia’s chances of breaking through the Ukrainian army and actually capturing that agricultural land are rather low even if Western support for Ukraine drops significantly, and Ukraine is going to be friendly to the West and hostile to Russia even if it isn’t allowed into NATO. If this scenario is intolerable to Russia, then whatever would happen is going to happen.
I do think there is a small but significant risk that Russia will use nuclear weapons in Ukraine (a scenario where both escalating and not escalating are likely to be disastrous) if its army is driven back to the border but not if the war becomes a frozen conflict with Russia controlling the territory it currently does. With that said, I disagree that shows of strength don’t deter. Western strength deterred a Soviet invasion of Europe, and it deters a Chinese invasion of Taiwan. There definitely is a risk of escalation, but there always will be. The USA has tried being isolationist before, but it was still drawn into both world wars. It will be drawn into the next one if such a war happens.