Ultimately, I don’t think that’s necessarily a bad thing. Democracy would be ideal, but I’d rather see Russia utterly dependent on Beijing than autocratic and warmongering as they are now.
I still fail to see the existential threat to China though. Even if Russia launches nukes, any retaliation is going to be hitting Eastern Europe which is very far away from China.
For all its bluster, North Korea isn’t invading anyone. China’s leash has kept the peace. Contrast that with Russia. It’s a fair bet that Putin (and his successors) will behave better under Beijing’s thumb.
As for why China doesn't give two hoots about Russia's military escapades, Russia's military focus is far enough away that this is significantly less problematic. Russia is also a much more serious player on the world stage than North Korea - they have a significantly more important economy, military, more technologically advanced, and not as isolated (not to mention one of the largest nuclear arsenals with ostensibly functional ICBMs). China can't just threaten to overthrow the Russian government if it doesn't behave like they do with North Korea.
None of that matters though since what I said is that an aggressive Russia benefits China well enough in terms of the problems it causes the US.
Which is why I think concerns about their aggression in the future are just propaganda. Russia can barely bully the poorest country in Europe. It definitively can't fight NATO.