The majority of Russian natural gas, oil, and coal reserves, not to mention iron and nickel refineries, are roughly halfway and have same distance to Europe and China. In theory you can just build oil and gas pipelines into Northeastern China. And in fact you already have ESPO oil pipeline and Power of Siberia for Far Eastern resources. It is a matter of time until the resources in the middle of the country get sent into China via pipeline as well.
Russia is vastly expanding its railway capacity into the Far East by expanding the railways by digging new tunnels, and switching to double stacked, double track railway freight lines in the Tran-Siberian Railway and BAM. At the same time they have plans to switch rail freight to cars which can travel at twice the speed from 80 kph to 160 kph.
Then you have the plans to have the Arctic open for marine traffic all year around once the Leader icebreaker class enters service.
China is trying to reduce overall oil and coal imports in the medium-term.
China already mines a lot of iron domestically. Transporting iron ore doesn't work out when compared to this. You can run the numbers yourself.
Rail transport is just so much more expensive than sea freight or a shirt distance truck haul.
Yes, the Russians can double-stack and double-track railways, but it is still more expensive and limited in capacity.
Arctic sea freight will be a big change yes, but this will take some years to happen.
And to reach China, Russian ships will still have to undertake a long journey around the Bering Strait and pass Japan.
That just leaves Natural Gas and Nickel which you've mentioned.