this is clearly half-baked and inefficient

No it's great! Lava hardens into rock, so if you shape/form/mould it when it is lava, then it solidifies into the shape you want.

There are lots of ways of making things that rely on the thermoplastic effect: you shape it when it's liquid, then it hardens and you're done.

If the wizard could bend/conjure lava into any shape, he would be a useful man to have on your construction crew.