Microvawe?

I believe the voltage is still high

I believe there are devices that can measure such things

I don't have any improvements to offer but I want to mention that I saw a crazy video that used a circuit breaker as a spot welding device. The concept is pretty much right and it is as cheap as it can gets, but it is electroboom kind of crazy and the guy even hurt himself on camera while using it.

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.

Are you able to measure the input current during a weld?

I suspect you might not have enough copper in the secondary. Fully insulated wires take up a lot of space; there's a reason magnet wire is commonly used. Several parallel turns of e.g. 6mm^2 magnet wire would be preferable.

Your SSR generally should be in the phase, not the neutral.

If it's a 240v transformer, the primary will have less turns than a 120v one. You may not have a high enough voltage out of your new secondary because of that.