We went to Universal the last week of August last year. We stayed on-site and had express passes. Most of the Harry Potter things aren’t eligible for express pass though. It was really nice to have it, but I’m sure you can do it without.
Have you tried making personalized Touring Plans yet? I think they give you a really good idea of wait times for particular days and specific times of day. Maybe make a few plans for different scenarios - all HP in one day, HP in the evenings, one park per morning, hopping mid-day, etc. It’s interesting to see how the wait times change. You’ll have to make separate plans for each park, by the way. You can adjust the start and end times and entry points to match your plan.
I found that the advice in the TP Universal book was spot on accurate. The shortest lines for the Harry Potter rides were at the very end of the day. The areas seemed packed in the mornings, especially right after the early entry time ended. At the end of August, when we were there, the parks closed quite early, like at 7 or 8, so I guess people leave for dinner and don’t return. Personally, I’d probably plan to do all the non-HP stuff in the mornings, and then save the HP areas for the evenings. I’m sure your plan to do HP all in one day will work too though!
The single rider lines are super fast any time of day, so take advantage of those if you can. You won’t be separated until the actual ride loading. (Gringott’s has a single rider line.) I’m sure you’ll be fine without express passes, but you can buy them at any time, so if you find the lines unbearable, keep that in mind.
The parks open earlier than officially posted. For the early entry park, they let everyone in the front gates and hold people at various spots inside the park. They only check for room keys at the entrance to the early entry HP area. So you can position yourself inside the park earlier than you might expect. Arriving right at 9 (or whenever park opening officially is) will get you there after a bunch of people. I would start the day at the park that does not have early entry. You can get a lot done the first couple hours of the day.
I hope this helps!