Yeah, just gonna chime in here real quick.
I went down this road just as you. I found out that most MOTs are rather weak for 18650 nickel strips.
You need a transformer rated around 1500 watts. Most are 700-900 watts. I ended up wiring 2 transformers in parallel. Also, make sure to remove the transformer’s shunts. They are a form of current limiting and will impact your amps at the end.
Finally, make sure to carefully regulate your pressure with the copper tips. High pressure does indeed equal a weaker weld.
AC in general is also not the best for very short pulses of welds. I have found that 40-60ms work best for 0.2 with around 1000amps. Anything less didn’t weld tbh and the MOT couldn’t pull amps fast enough. I tried all sorts of windings and cable thicknesses. I finally chose 6AWG and I’m happy enough with 2 transformers.
The smoke is from the rosin core and flux. Lead free is equally bad for the lungs.
I wouldn’t eat after touching Lead though.