My GM brain also instantly thought the swap would be great at around level 6. It's the sweet spot of experiencing the first character pretty well while also not putting the swap so late that it becomes difficult.

Also I think hiding the backstory for a bit seems like a great move. Because as it happened on your game as well, that's the type of backstory that once the group knows it basically becomes a high priority. But sounds like it all worked out amazingly. Your group appears to be a great fit for each other with your styles of play.