For the uninitiated, generally NSFW is for sexual contents and NSFL is for gory contents. People may want to see one but not see the other at any time for any reason. I have seen this feature requested over the years in Reddit but it never happens. Maybe now some instance can finally implement it?
Yeah, it'd be better to implement OP's idea, see how it goes, and then see if there's need for the others. Gradual changes help admins see what works best, I think. Follow the KISS: Keep It Simple, Supid.