Is your express from staying at a premier hotel? If so, then don’t worry about the crowds. On Sunday, you can plan to go back to the hotel in the afternoon if the crowds are too much. Take a nap, chill by the pool, whatever you want to do for a rest. Have dinner out of the park. Then go back in after dinner to enjoy some after-dark park time, including the dark arts at Hogwarts stuff.
As for sleeping in, go for it! The IoA ropedrop is insane for Hagrid’s coaster, so if you’re not at the front of the pack it’s a two hour wait very fast. Better to go later, use your express on other things, and then, if you want to ride Hagrid’s, do it after lunch. Waits tend to dip slightly in the afternoon, but even if you do wait 70 minutes, it will be your only real wait of the day (Pteranadon flyers also does not accept express, but it doesn’t get 2-3 hour waits). With express and only one park, you want need to be there open to close. On Monday, you’ll be at US, and all the rides there take express passes, so there’s really no advantage to rushing over.