Russia needs to be able to produce enough weapons to wage war. That is also true for any potential war against EU members, such as the Baltic states or Poland.
Also Russia and Ukraine can both be defeated in the same peace deal. Just look at the British Empire and WW2. The UK one, but it lost its Empire.
There are a lot of countries, which Russia wants to invade besides Ukraine. I mentioned a few and there are a lot more in the Caucasus and Central Asia. Even Ukraine would benefit, even in a total defeat. That would mean underground warfare against Russia and a weak Russia would be hurt badly by that. Ukraine is just much harder to control then Chechnya, if they do not want to be a part of Russia.
No, they mostly just trade with Russia. They are not sending aid. If Russia collapses Iran and North Korea would be unhappy, but they will not intervene. Turkey and China do not care at all. Both of them can benefit from it, if they play it right. I am just saying Central Asia and for China parts of the Russian Far East.
For example one, which freezes the current situation. Russia looses, as a victory was always to take all of Ukraine or at least the territories it claims to be Russian, which it does not control and Kursk. Ukraine also looses as it has already claimed victory would be taking back control over all territory, which is internationaly recognised as being Ukrainian. So both sides have been defeated. That is actually a very likely outcome.