I've talked to many sex workers and third parties who are being double-charged with sex work and human trafficking offences. I think it's really problematic to suggest that the low conviction rates are because it's hard to prove human trafficking. They are because police and prosecutors are charging people and expecting people to plead to the charge of sex work. We see that consistently in so many cases, that these charges are eventually withdrawn because people will plead to a sex work offence.
I think you need to reconsider the idea that it's very hard to prove. It's actually because of police and prosecutorial practice.