We are typically budget travelers, but can't say enough about staying onsite at universal. With express you can loop your favorite rides as many times as you like. The only waits will be the shows where you have to wait for the prior show to finish and clear out. You still get express entry for those. The HP rides that do not have express can be ridden once or twice during early entry with little wait.
As far as sleeping five: The rollaway will take up most of the available space in the room, but chances are you won't be "in" the room that much so it's workable.
You've got some time before your trip, so you might look into credit card reward programs. We accepted an offer from Blue Sky rewards (Am Ex). The sign-on bonus was enough for one night at Royal Pacific Resort. We completed two sign-ups (one for DH, one for me). I believe we had to put $500 in expenses on each card (which we did by running our regular expenses through the cards and paying off monthly so no added cost to us) and we earned the sign-on/bonus rewards... Redeeming the rewards for the RPR was super easy through the Blue Sky website. It was well worth it!
If you stay at Cabana Bay for just early entry, you can still make it work, it just won't be as effortless (we visited Christmas week pre-potter and in July post-potter). The lines do get long quickly for the newest attractions and the main coasters. There are single rider lines if your family is willing to split up. That will help on some of the headliners. And try not to carry a lot of stuff (or use a fanny pack with a three-prong catch which is legal on the rides), it will save time if you don't have to put everything in a locker for each major attraction.
Have a great trip!
