This was not what I was trying to print, but it’s what I found on the printer this morning.
Today, as a result, I replaced the thermistor, heater cartridge, throat, nozzle, heater block, and throat lock screws on my printer. I also replaced the cheap 24V fan with a 12V noctua and a stepdown converter while I was at it. Getting hardened PETG off apparently requires THF which I don’t have on hand, so I just threw out the heater block, throat, and nozzle that I couldn’t get clean with just heat.
The thermistor pulled partway out of its can, and the surface of both the heater cartridge and thermister can were coated with melted PETG; it apparently injected everywhere under pressure.
The replacement heat block has a silicone sock instead of being wrapped in layers of kapton tape and I expected to have to pid tune again, but the temperature already holds better even without a pid tune cycle, so I’m back to printing. I hope this one works better.