we mostly use coper wire and nickel strip.
coper strip is impossible to spot weld in a DIY sense.
some are playing with nickel plated coper strips. but it’s much less common.
I suggest use nickel to join the pgropus. ideally one big piece of nickel. use coper wire to join th series group and make final attachments. if you can scratch up and “pre-tin” the nickel with solder before you attach it. (avoids heating/damaging cells from soldering heat )
check out the battery builders club thread. It’s giant. so follow it and read the latest. but also jump back randomly some time and read peoples builds and the in particular the critques on them. post your own progress.