Soldering won't do any good unless you have a known good backup (and ntrboot will help in this case).
The contents of the NAND chip is completely irrelevant for ntrboot.
Problem with ntrboot is that it can fail for many reasons and the result looks always the same:
- Bad SD
- filesystem is not FAT32
- filesystem is damaged
- card is corrupted
- card is fake
- card is defective
- Bad SD reader in the console
- Magnet
- placed wrong
- too weak (but don't use some insanely strong thing!)
- →→ Old series consoles will not turn on when pressing (Power) only while magnet is positioned correctly
- Wrong file
- No boot.firm
- boot.firm not a valid boot file
- Gamecart reader
- reader bad
- contact issue with the R4 (they aren't as precise as the original games)
- Holding buttons wrong
- wrong combination
- holding too short period of time
- one or more button(s) does not work
- →→ I got the best results releasing (Power) first
- →→ Old series consoles are more stubborn in my experience
It takes some practice to get it right. I'd use a working console of the same model for this reason. You can rule out a lot of the list above when succeeding on another unit.
Boot file should be renamed GodMode9. This gives the most possibilities (including booting into SafeB9SInstaller or trying to install FIRM with GodMode9 itself).