Yes agree for up to 20a pure nickel is all that is needed, just a case of using correct size for cells in P to series to carry the amperage across.With those currents I can see your point about using copper. For those of us using 15-17 Amp controllers I think pure nickel will work just fine.
In the experimental X pattern for parallel I used, parallel busses only need to be able to deliver the individual cell current to the center, it is the series buss that needs to handle the combined amperage.
Generic China battery with branded cells tend to use 8mm - 12mm x 0.15 nickel sometimes layered.
Really cheap unbranded cells I don't know but on ES some use plated steel and pics show the rusting.
Bosch builds are quality and use bespoke laser cut buss and can be 0.2 or 0.3mm nickel x about 12mm.
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