So I should try it with flashcard and ntrboot?
Yep you can use an NTRboot flashcart to install B9S, follow instructions here:
https://3ds.hacks.guide/ntrboot
You'll need a compatible flashcart and another DS which supports the flashcart, or you can buy a flashcart with NTRboot preinstalled.
After you install B9S, follow this:
https://3ds.hacks.guide/ctrtransfer.html
Or I think you can also use your NAND backup?
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