Ok, I bought the new Opi 3B version (aka 2.1) and DietPi can boot without problem, and my NVME SSD is working, so the problem is my board. I really don’t know if DietPi is the culprit, I believe it’s not.
Strange enough, I got DietPi working in my defective board doing these steps:
1 - Compile the official Opi 3b uboot following the steps from:
2 - Save the file somewhere.
3 - Burn the official Opi Linux into eMMC
4 - Burn the DietPi image into sd card
5 - Be prepared to insert the sdcard into Opi 3B, but don’t do it yet
6- Starting boot the Opi 3B from NVME and after 2 seconds or soo, quickly insert the SD Card containing DietPi
7 - That way, DietPi will start with the Opi official linux bootloader.
8 - Now install the uboot file you just compiled into DietPi
9 - using the dietpi menu, burn the uboot into sd card
10 - now you have a bootable dietpi in a defective board
I believe the Opi team knows about this problem, that’s why their uboot works every time, and maybe that’s why they made a new board version.
See reference about the NVME problem:
Orange pi 3b NVME problem - Troubleshooting - DietPi Community Forum
Also, in the new Opi 3B board, ethernet is not working with DietPi (that’s another thing).