Imho encrypting the phone once, which fills the storage with a pseudorandom blob of data, and then factory resetting is sufficient. In fact I would remove the SIM card and then factory reset just to make sure that any information from the sim didn't leak to the phone would also be fine.
Flash storage is not like a spinning hard drive, where previous data could leak without several attempts to flip all of the data bits randomly on the drive. There may still be some data, but it would a trivially small amount.