My conception was that we're adding power by increasing the voltage and we're stacking up material by adding wire, so to get good penetration on thick material I want lots of voltage and not much wire feed.
But actually it turns out that you make it hotter by increasing the wire feed speed.
I don't really see why, but at least now I know.
Here's a more recent weld: https://img.incoherency.co.uk/6464 - still not great but not nearly as bad.