Hello Ashley,
Now obviously everyone is going to have their own experiences and advice so feel free to plan your trip your way. Last week before heading out of town I took a few family members from Seattle, DC, and Philly to Universal. Some had never been to Universal ever and some just hadn't been to Diagon Alley yet. Given my knowledge of the parks I planned the day as follows and it worked VERY WELL.
We missed early entry due to late flights coming in the night before but hit the parks right at 930. We went directly to Marvel did the Hulk twice, then Doom, and then Spidey. We then shot over to Toon but skipped Popeye and then skipped JPRA due to temperatures. We then hit Hogsmeade and did FJ, Ollivanders, and checked out all the shops. After that we did all of Seuss and then shot back to the Hogwarts Express. The longest line we've encountered at this point was 20 mins for FJ. Once inside USF we met up with another group knocked out MIB, Springfield (Ate) , ET, Transformers, Twister, Mummy, and then saved Diagon Alley for last. The group was very happy we saved Diagon for last and Gringotts was only a 30 min wait. We spent the rest of the day there and in London taking it all in. They LOVED IT!! I had to fly out the next morning while they all stayed and finished everything else the second day and spent another 4 hours or so in Diagon.
Now I've done it the other way also with first timers and it didn't work out as good. We headed to Diagon BEFORE Rope drop and upon arriving the line for Gringotts was already 90 mins due to early entry hotel guests all heading there. By the time we finished a majority of Diagon's offerings we noticed wait times were increasing across the park so we headed to IOA via the HE. When we arrived at IOA by close to noon all the major attractions were at 45-90 mins each. We weren't able to do nearly as much as I had hoped for the first timers. I was a little embarrassed the park I raved about wasn't really able to be experienced due to poor planning. But hey you live and you learn right. So thats why I did it different this time and it worked flawlessly.
Hope these previous experiences of mine helped. Its close to 1AM here and just got off a 10 hour shift about 90 mins ago so my apologies if I ran off course or misspelled a few things lol. If you have any specific questions please feel free to reach out, I love talking Diagon, IOA, and ANYTHING UNIVERSAL!!