According to Nikon, the NEF format supports lossy, lossless or no compression (there's a setting in newer cameras). What I have is a terabyte of NEFs from an old camera which are uncompressed. Is there any way to convert them to the lossless NEF format now? Or any other lossless compression method I can use, similar to JPEG XL?
I do not thing there is utility to convert between compression schemas (actually different formats) for NEF files. You can test Adobe DNG Converter if the resulting DNG is smaller by size (but I doubt).
The other way is to use some file compression utility like ZIP, RAR, etc to try to make them smaller. But this will add additional step in your workflow (to decompress the RAW files).
As general recommendation: keep them as they are, do not make any changes on them. Any change in RAW file may lead now or in the future of broke something or make them unreadable. Moreover currently the price of storage decrease so add more storage and keep the originals.
About backup you can use Amazon Photos which offer you unlimited space for photos (including RAW files). I currently keep there almost 3TB of data.
I was able to get some results with Adobe DNG Converter, default lossless settings:
I get a 10% file size reduction with my photos, so 20MB uncompressed NEF > 18MB DNG. May not seem like much, but that's already 100GB+ saved in local storage, backup storage and bandwidth.
Note: while pixel data is prerserved 1 to 1, it is possible that some camera metadata will be lost by converting (depending on your camera and Adobe's support for it).
For anyone else on Linux, the tool runs perfectly on Wine.