Tried as you suggested. I had only an external HDD hooked in the USB 3.0 port in the front, and that HDD has its own power supply, so I don’t think it was drawing any power over USB.
I unplugged it, also unplugged the eth cable for good measure, but it’s still the same issue. XU4 turns on, the fan speed ramps up, and then does nothing.
It works fine booting with the Arch Linux ARM setup I have and the external HDD plugged in (actually, it doesn’t boot at all if it’s not plugged in as the external HDD has the home partition), so I’m not sure if it’s a power issue.
Also I have the 6A power supply with this XU4.
Unfortunately I don’t have another power supply to try out with this.
Anything else you could suggest I could try?
EDIT: the Arch Linux ARM also has a separate boot partition like the old version of DietPi had.
EDIT2: checked the sum of the image with sha256sum before I wrote the image to the uSD, it was valid, also let Balena Etcher verify the image it has wrote.