Nicely done. I would be interested in seeing the schematics and overall design too!
What is its upper limit welding wise?
Have you considered using tungsten rods for welding electrodes? They take much higher temperatures before they weld themselves to anything.
I made a set for may Miller 2.5 KVA resistance spot welder. I run it with a variac and drop the current and voltage and then focus the spot weld further with the tungsten rods sharpened to a dull point.
I just took a worn out set of copper electrodes and drilled holes in the ends and silver brazed the press fit tungsten points into place.
But I cant do anything near as small as your showing!
But on the upside Mine is 240 volt powered and can spot weld three 10 gauge sheets together at once!