Trace width in small PCB board designed to combine battery cells in series
Why don't you just flip all the odd cells?

If you flip them, you can connect them with a much shorter and wider path.

You say the yellow traces are 5 mm wide, I estimate they are 15 mm long. If you keep the exact same landing pattern, but flip every other cell, you can connect the cells with traces approximately 20 mm wide, 3 mm long. That is like 20x better (lower) resistance.