Hi, I've had a similar problem in the past, and the best way was a "Hardware NAND Restore", I got a lot of help from GBATEMP users and it worked, the hardest for me was the CLK soldering, but I got a standard O3DS, and O3DS XL CLK is much easy to solder. The most impotent things are:
1. A "MMC" SD CARD reader.
2. Cables as short ass possible, and they can't be soldiered to the SD ADAPTER directly, there has to be a connector in between (That was my biggest overlook, but after I cut the wires and did that connector, it connected and I restored, my NAND).
3. After launching "win32diskimager"
DON'T PRESS FORMAT DISK!
Here are some links:
http://gbatemp.net/threads/nand-flash-dump-3ds-xl.350668/
https://www.reboot.ms/forum/threads...3ds-xl-nand-dump-ripristino-via-hardware.207/
Good luck