I never been on an crowded Far Rock or Hempstead train outside of rush hour, once an hour off peak is sufficient IMO.
For whatever reason, Jones Beach is a more popular beach destination than Long Beach. To me Long Beach is nicer than Jones Beach, it's right in a beach town where's Jones Beach is in the middle of nowhere. Hour headways on Long Beach is fine.
A few July 4th's ago (pre pandemic) I rode one of those Long Beach super expresses in the evening and the train wasn't busy at all.