EXIF Film Data for slides

I need to scan at least 10K negatives and about 20K or more slides. I like that I can use NLP for film EXIF data, but I would also like to use NLP for all my slides. Is that possible

Yes, you can the metadata fields that Negative Lab Pro adds to Lightroom Classic, even on slide film. In this case, there is no need to open NLP on your slide film. You can just add the metadata directly. The one caveat is the metadata will only live in the Lightroom Catalog until you export it. This is done to avoid conflicts with Lightroom’s need to access exif metadata on the raw photo to know how to render the file.