While I understand the logic wrt the concerns about the content of hentai, I do find it interesting that it's so prominent in the discussion of safety issues.
To me, it's always felt "safer" than real-life content because a lot of the big risks go away. I don't have to consider whether the actors were coerced, or whether they would have been able to stop a scene if uncomfortable, or whether they regret putting that content out there and so on.
As a consumer of hentai or similar, it becomes a lot more reasonable to say, "I don't know what the imaginary background of this character is, but I'm interpretive them as an adult, so I'm all good", or "Did they really give their consent to dick-cthulhu? Of course they did! Who wouldn't!?" because I can't really be meaningfully wrong about a imaginary character.
Whatever the morality of, I guess let's say, fictional immorality, the potential harm from "real" porn just seems so much larger than the potential harm from drawings and writing. However much I enjoy seeing real human beings doing delightful things to each other, if the only porn on the internet was hentai and dirty stories, I'm inclined to think it'd probably be an overall win in terms of harm reduction, just because it doesn't require real people to be doing the stunts. So it's interesting that the fictional stuff seems to be so top of mind when we talk about safety.
Although, I imagine that's likely because in the discussions of rule-setting the issues around "real" porn are talked about far less, because who's really going to make a good-faith argument that's pro sharing images of abuse of real people.
(Also found your point about cultural imperialism interesting! An angle on the topic I'd not considered before)