You guys and gals are geniuses. Sure enough, it turns out I had flux core wire in the welder which meant two things - the polarity was wrong, and I was using gas when I didn’t need to. I bought some solid wire during my lunch break, and gave it a quick test tonight:

That’s definitely looks better! I still have a bit of a wire feed issue, though. I picked up some new contact tips, but the one didn’t want to screw in nicely - I think the threads may be slightly messed up on the torch.

Also, I learned another important lesson: no exposed skin! Last night I was welding just with a helmet, gloves, and some sleeves, and got a sunburn on the inside of my elbow. Tonight I wore a jean jacket and gloves, but no sleeves, and still managed to catch a tiny sunburn between the glove and the sleeve of my jacked.  Mental note made - always wear everything!