Hmm, china currently thinks it can get 60% of iron need from domestic production, steel scraps and ownership of foreign mine. If there is a decoupling, china will really be depending on both Russia and Afghanistan. It also needs Russia for copper. I bet there are a lot more stuff in Siberia that they haven't discovered yet. As green energy continue to gain importance, I think there will be more exploration effort toward finding stuff like lithium, iron, rare earth and copper in Russia.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.