my thoughts after i got mecha 7:
"yay, i got min!"
"...wait"
"oh dear god no"

very good metric, i like it. every input goes through a bonder/debonder that breaks off an air/salt pair and adds the remaining salt to a waste chain. this let us construct the large output 2 atoms at a time. however, the output has an extra air. to add this extra atom, we instead add 4, and debond the excess 3 to be carried away with the waste. at some points, the waste chain gets terrifyingly close to colliding with the product, but thankfully it doesn’t. unfortunately i had to 6p this solve since there are 4 cycles of setup at the start. i think mecha 5 is almost certainly possible, but i am not surprised that i could not find it.