I’m using nickel strip for my packs. I have a continuous roll. I’m not sure if it is really only nickel or nickel coated steel (e.g. hilumin by tata steel). The steel ones probably have higher resistance, but I don’t care too much as the current between cells in parallel should be very low anyway.
I would recomment first soldering and then spot welding to the cells, too.
I’m not sure about your setup, but I can spot weld another strip of nickel over the first one to decrease resistance. I soldered a solid copper rod to the last nickel strips, this collects all the current from the parallel cells.
I would recommend testing the spot welds on some old lapop pc cells or other test cells. Fine tuing the settings and the pressure with the hand tool really pays off. Too low pressure on the contacts and you throw molten metal around, with too high pressure you deform the battery housing…
With the test cells I usally made 4 test spots on the positive side to attach a nickel strip. You can then pull of the nickel strip. It should really leave holes in the nickel and parts of nickel on the battery.
Do not use the negative side of the battery for tests as a failed spot weld or when ripping of the nickel from a good one can leave holes in the metal battery can. This exposes the inner fluids