We had APs at Uni back in Sept 2022 - Sept 2023, so made several trips. There is almost no planning needed after you book a hotel with Express Pass. I didn't have kids as young as yours, but there is even more for the littles now because they finished Minions "land" and rethemed the kids area to Dreamworks. IMO, he's a great age to make a first Uni trip. Spiderman is still a great ride after all these years. We thought Transformers, which is the same type ride, is also really good, even though we aren't Transformers fans. Even meeting the Transformers was cool! He might like Men In Black too, which is a fun Buzz like ride. If you and hubby are coaster fans, Hagrids is truly worth the wait if you can catch it when the lines aren't too bad. Do not do it at RD unless you are one of the first families into the park.
The Park to Park is not worth it only for the Hogwarts Express, unless you have a huge HP fan. However, it is nice to have because it is so easy to park hop with both parks next to each other. It's like park hopping at DL.
I think Hard Rock is in the process of a soft goods refurb. That may account for the weird room price differences you found. I've stayed at HR but not club. Loved the location. We did club at Portofino and it was great.
I've done PE back from AMS on KLM. I've also done PE on United a few times for longer flights and Delta for one. I wouldn't hesitate to do PE for a day flight on any of them. Especially a NY or BOS flight to London, which isn't a very long flight.