After accidentally formatting the wrong SSD my first course of action was to try a recovery using Recuva. Recuva did manage to find and apparently recover 4.943 files, but only 224 jpgs were readable. Most jpgs, and all NEF/DNG-files with their cab-files are not recognized by Windows Photo Viewer or Photoshop CS6 and cannot be opened. No tiff-files were recovered.
Is there an alternative solution, or can I just say goodbye to the files that were not backed up?
Recuva can't repair files; it can only recover the files exactly as it's reading them.
As such, there's no options in Recuva to change how it recovers the files. If it detected and attempted recovery of the files, then it's done all it can. (SSDs are notoriously hard to recover from.)
You may be able to use something like IrfanView: https://www.irfanview.com/ to open the image files; it can work around some kinds of damage to open the files, wherein they can then be resaved 'cleanly'.
I see a few tools claiming to be able to 'fix' NEF files like: https://exiftool.org/fix_corrupted_nef.html or https://www.nefrepair.com/ but I've not made use of these tools (or indeed the NEF or DNG formats at all), so I'm unable to offer more information about them than their existence.