Why would a billionaire being surrounded by young woman be a red flag? Being a billionaire is enough to explain their presence without any hint of coercion.
Because it was supposed to be a professional event and that's not even remotely professional? There's a second hand account in this thread by someone who knows people who went to the island and were propositioned for sex there.
It's unprofessional at best but as we know know, many of these girls were sex trafficking victims and underage.
It's also known that Epstein's island residence was decorated with pictures of underage nude girls and a giant nude painting of Ghlisaine Maxwell. So yeah, I'd say there are quite a few red flags there.
I think the concept of "professional event" has changed considerably in the last few decades. So judging what should or shouldn't have been known by the guests based on modern standards is a mistake.
As far as nude pictures and paintings, were they clearly underage and non artistic? There's a lot of post hoc reasoning by those who want to spread blame around that doesn't stand up to scrutiny.