Just returned from a 5 night/6 day stay on site at PBH. We did't go to any earl yours since we had heard the stories of mounds of people waiting and waiting. Here is what DD (17) and I did and it worked well, at least for us.
1 Go to IOA first and do stuff there and of course FJ and Harry Potter. We usually slept in , ate lunch and then went around 1:00 or so.
2. Ride Hogwarts Express from IOA to Diagon Alley at US, early evening 5 or 6 pm.
3. Do a few things at US and then go to Diagon Alley. Less crowded, no waits for the stores or at least very small waits! We walked right in at Ollivanders! Ice cream shop small wait!
4. Ride or at least get in line for Gringotts BEFORE 7 or 7:30- They close the line very early because of the so called wait! We tried to wait until 8 one night when they closed at 10 and couldn't! Single line is much shorter so maybe second ride you might pick that option if you don't have small children.
5. Go later for less crowds and same fun! Usually cooler too!
6 . Have fun and enjoy!