5 Years ago, I designed and built a 12V battery based on cells sold at battery hookup. It's been running every day since October 17 2021. The cells are 3.2V 600mAh. I have a battery balancer attached to the cells which very rarely illuminates. The arrangement is 9 in parallel, 4 Series so 4S9P. It works great in winter and summer, no issues at all. If you are planning on doing something like this, you can definitely do it on your Prius. I cannot say the same for other cars. I would not jumper other cars from a battery like this either. But anyway, screw lead acid batteries, they never last 2 years in Washington State weather. Here I have proven that LifePo4 can do 5 years without any issues. The battery looks like new on the outside and on the inside. It changes fine and the car has not stalled or left me anywhere stranded since. Original post a the other place before I was banned for whatever stupid reason. https://www.reddit.com/r/prius/comments/qacx7w/im_testing_my_32650_12v_lifepo4_battery_after/
I too took a welding class at my nearby community college, and it's an awesome skill to have. That said, my understanding is that spot-welding for battery cells is substantially easily than TIG, MIG, stick, or any of the other large-format welding processes. Yes, PPE is still highly advised, but spot welding seems like something which can be picked up through watching a video.
Not to simplify too much, but a spot welder has only a few controls: the location of the weld, the current in amps, and the weld time in fractions of seconds. Some hobbyist tools specifically for battery packs will automatically execute a weld for the precise amps and seconds, provided that you make contact with the electrodes. Others have a foot pedal to start the weld.
Thank you for the encouragement, but I am a scardey cat and would feel better getting better training than YouTube