I agree with you that a formal security alliance is inconceivable, for two reasons:
1) What does China have to gain from such an alliance? China can never overtly support Russian separatist movements in Russian satellite states without inviting the same applied to its own peripheral territories (e.g. Xinjiang, Tibet). It's beyond belief that China would spend blood and treasure to defend Russian territory. At the same time, how could Russia possibly help China in any material way?
This I see no problem, we can easily have our cake and eat it too. We can support Russian advancing on Ukraine and the Tibetans and what not can just deal with it, cause guess what, while they are not completely happy with the government, name me one guy that is, they also don't hate it enough to separate.
The minority always scream the loudest.
Ask yourself a logical question, how likely are Tibetans to find happiness in India, knowing what India actually is.
2) Russia and China compete for influence with the other members of the SCO; Russia through its USSR links, China through its Silk Road links. There is more likely to be friction between the two over Central Asia than there would be joint interests elsewhere (China has very strong trade links with Europe that it would not risk harming on Russia's behalf; Russia has no reason to downgrade relations with the likes of India or Vietnam for the sake of ephemeral gains from China).
Moreover, they increasingly compete on the defense industry front, and they don't share the same historical outlook on the West. Once the CCP decides to switch the artificial narrative of how the US is trying to contain China to the reality that the US helped China in WWII against Japan, and favored China over the USSR from the 1970s onward, the unity of purpose between Russia and China crumbles.
This is something I agree and is often overlooked for stupid reasons, like the fact US don't like to share and hate anything not created by it.
China and US can do good for the world, China is all about the growth and in terms of rights, we are far closer to Gay marriage than even democratic countries. Reason is while not democratic, we are not religious, anything that doesn't effect development is cool with us.
While Russia and China actually have competing interests, I mean actual core interests like central Asia, Pacific is only very important, and hardly damaging to the US if lost. We won't build an great wall around Asia, cause we like your money.
If US can work it out with itself, that China's rise is going to happen whether you like it or not and that we are actually not an empire of evil, our friendship could be for the greater good.