I encountered same trouble. "boot.firm.bak" file created on SDroot. I rename to "boot.firm" and delete new same neme file, and then my 3DS was revived!
If you use lumaupdater, you should update B9S.
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day