Trade In

Being even less trusting than you when it comes to Apple’s trade-in partners I would start the photo sequence with a pic of the watch before it was wiped, showing the About screen and particularly the serial number. Then the regular watch face showing date and time to X-ref against the photo metadata.

I'm not seeing why having a photo of the serial number screen is important. The photo of the watch face is fine but I also do a video changing screens and running an app to show it was actually working.

No, just the Watch. Just before you put it in the box to send it take a short video of it actually working and many date stamped pictures of it from all angles and save them.