If uncertain about whether something is worth the nsfw tag or not, I usually suggest a "safe" thumbnail, plastering the title and your leading text with warnings and if someone is dumb enough to click in anyway that's their problem ;D

It wasn't any worse than stuff they've seen already (and definitely a lot tamer than some of the R18+ anime my 13yo was apparently watching at a friend's place x_x I don't know if the parents allow it or if they didn't know either because cartoons are for kids right x_x)

I can't tell you if you've found the edge or not because I'm the kind of person who won't notice if you answer the door in your undies unless you mention it first or it's really cold (an actual thing that has happened).