Why Russia CANNOT accept ANY peace-treaty that allows Ukraine to survive as a country:
This war is not about territory or the West supposedly being a military threat to Russia.
This war is about the Russian Federation's geopolitical claim to cultural monopoly that THEY AND ONLY THEY GET TO DECIDE HOW SLAVIC PEOPLE ARE SUPPOSED TO LIVE.
All neighbours of the Russian Federation look up to them. They want to be like them, they want a part of their wealth and power. And this gives the Russian Federation POWER over it's neighbours.
Now imagine a situation where there is a russian-speaking country that is wealthy and successfull while politically being totally unlike the Russian Federation.
The mere existence of this other country would destroy the geopolitical narrative that the Russian Federation's way is THE ONLY way.
That's why the existence of Ukraine cannot be tolerated. Because it would set a dangerous precedent that obeying the Russian Federation is actually not necessary for economic wealth.
2 months before the Ukraine-war there were massive anti-Russia protests in Kazakhstan, an absolutely crucial geopolitical ally of the Russian Federation. If Ukraine sets the precedent, then Kazakhstan will next abandon the Russian Federation, most likely for a treaty with China, and that loss would geopolitically be absolutely colossal.