Sanu has the advantage of being signed for 2020.
Green, Sanders and Hooper are rentals. Essentially, the patriots would need to hope to sign a new contract. For me, these players only make sense if the contract has been already negotiated. The teams could allow these players to negotiate; it is possible. However, since all these players have more than one likely suitors, the current team might want to simply have the agents not deal with other teams, so that they can do their own negotiations.
Folks don't like Sanu because we already have his skill set in others. For me, that matters little.
Sanu would be our #2 WR. Josh would be able to make him value in game plans.