Sorry, I forgot to update this.
Turns out it wasn’t a relay. After a 1.5 hour drive the exact same thing happened.
Connected ISTA+ and the DDE was offline again. The DDE didn’t come back online that evening either. Previously it was back up after 10-15 mins.

Had it towed back to the dealer and apparently it’s now starting and not throwing any codes again. Good times. Will update when I know more.

It can only be electrical. I wonder if the DDE is slowly failing and turning itself off after a period of time.