Just got back... tip if you have two people... we managed to get 6 day-of FP for FoP and 4 (could have gotten many more but how many boat rides can you do) for NRJ across two days. All were booked after using our initial 3 advanced FP. You just need to look during your downtime, I did it while waiting in lines, and just keep switching times. You do that for a couple minutes and there’s a good chance one will eventually pop up. Was literally in the FP queue for FoP and got another starting 10 minutes later.
Your mileage may vary, especially if you have more in your group, but if you’re determined (you can’t just try for 10 seconds and give up!) two people can ride FoP/NRJ quite a few times.