Use TIFF as the export format. It’s lossless, so you avoid the quality loss of JPEG compression and keep as much image data as possible when handing the file off to Nik.

Shooting RAW is still very worthwhile even if Nik doesn’t work on the RAW file directly. RAW gives you more latitude for the first stage of editing in Lightroom, especially for exposure and white balance. A good workflow is to make your key global adjustments in Lightroom first, then export to Nik as a TIFF for the plugin-specific edits.

That way, RAW still provides the flexible starting point, and TIFF serves as the high-quality working file for Nik. You also keep the original RAW in case you want to re-edit later without Nik or with a different approach.

If you also use Photoshop, another option is sending the file there and using Nik in layers or smart objects for more control.