The border with Russia is all Siberian as you probably know. You should also know that a couple special forces strikes on the rail infrastructure and ... no grain for China. Ukraine has already carried out successful raids on Siberian infrastructure. Infrastructure/logisical lines in Siberia are incredibly vulnerable: they are expensive to lay down, they are geographically extremely extended, there is basically no way to defend it. The Siberian people are not exactly pro-Moscow either, especially since their young generation has been preferentially/racially culled for meat wave tactics in Ukraine.

It is doubtful what can be essentially considered a single fragile rail route can provide it logistically.

the Russian yields are not fantastic, and atop that Ukraine is for the foreseeable future not available for providing grain.

A war with China will disrupt international trade so much that grain demand will explode (countries will probably stockpile more) and the grain providers can select 1) blockaded china or 2) any of a hundred other customers.

https://chinapower.csis.org/china-food-security/

So Brazil, Argentina would be out, can't ship to China

US would obviously stop exports

Canada, Australia, New Zealand, France: out

Indonesia, Thailand: probably out.

I agree that Russia is not in a situation of imminent collapse, neither is Putin in danger of imminent overthrow. However, demographically the Russians are expending the last of their skilled and young workers, while forcing Europe to no longer need their oil + gas, accelerate all EV / PV / Wind initiatives. Russia will lose "the last decade" of high oil profits before the demand begins to collapse.

Russia has also emboldened its other satellite conquests with its military incompetence. There was a video of Putin negotiating publicly with one of the southern states (don't remember, may have been Georgia) where the leader at the public negotiation table outright defied Putin. Meanwhile Ukraine modernizes, trains, becomes stronger and more effective. NATO is arming, and at this point Ukraine will probably be the site of an actual NATO-Russia conventional battle, which given NATO's ability for total air superiority means a complete defeat. If you're wondering who would fight the ground war given likely American apathy for another war, Poland would ENTHUSIASTICALLY fully mobilize to fight the Russians.

Down the line, Belarus's propped up government will fall, and Russia is staring at virtually a multi thousand mile border with outright NATO or might-as-well-be-NATO countries between Finland, Baltic republics, and Ukraine, and probably Belarus as I just said.

Black Sea access is very tenuous. Kalingrad is unreachable, and may secede. North Sea access is really tenuous.

Honestly the only hope for Russia is Europe doesn't arm fast enough and the US House of Reps holds up funding (which I believe 96bln just got approved).

And Russia's leadership is historically aware, what is happening as a result of this war (severing of gas supply dependence) is an armed Germany. The prospect of a fully industrially armed and modernized German military should strike a deep cultural fear in the entire Russian people.

And we didn't address the oil/petroleum issue. China can stockpile what they can, but an embargo will still result in the lights going off in China within 30 days.

China is still essentially a warlord state, and Xi is likely generating substantial resentment in the regional power centers. If the central government fails in a blockade, the Chinese state, fragmented between the free market liberal urban governments who resent the free market rollbacks, the CCP power center, and who knows in the rural areas, will crumble into warlord states.

As a side note, not a lot of people talk about the SOuth Korean demographic bomb (worst in the world), but if China falters the solution to South Korean demographics is to topple the North Koreans and import their population into the South Korean economy. I think that will happen in the next 20 years.