Do you have a rough idea for when Braavos was founded? I’m particularly interested in whether it was before, or after, Nymeria’s flight from the Rhoyne. Thanks for the help.

Braavos was founded roughly a century before the Unmasking of Uthero, which we know happened before the Doom but we don’t know how long before the Doom (although we do know that Braavos is the youngest of the Free Cities). 

So Braavos was founded at least 200 BC. But given that Nymeria’s voyage took place in 700 BC, odds are that Braavos was founded after Nymeria’s Conquest.