It finally happened: I got my tortured Pi 400 to thermal throttle. It took longer than I expected. I've been rendering videos on and off all day and finally did it with Handbrake. It rendered a short 4K clip at 1080p, very high quality settings, and I got the little flashing thermometer symbol in the top right of the screen. The system never crashed so I'm ok with it. Active cooling is definitely coming. I want to get the system set up for daily use before I start worrying about extra hardware.

Anyway, I hit an interesting milestone. I' considered turning down the overclock from 2.2GHz to 2.0GHz, but I want to see what happens and how long the Pi lasts. I may order a backup/secondary Pi 400 soon anyway. Good stuff.