So far the official answer is After. As of Summer 2017:
According to Amy Pascal, the producer, Far From Home will start just immediately after the finale of Avengers: Endgame.
She added: “And we are starting now the next one which will start a few minutes after Avengers 4 wraps as a story.”
The full quote can be found in this Fandom article from June 2017:
“What I think we should focus on is this Spider-Man who started in Civil War and then has this movie, and then will be in the Avengers movie. And we are starting now the next one which will start a few minutes after Avengers 4 wraps as a story,” Pascal reveals.
That's not to say, of course, that they have not changed their mind, or will not change their minds.
Of course, now, we know that it's set afterwards because the trailer for Far from Home has
graffiti memorializing the deceased Tony Stark, who died during Endgame.