Once the parks can go back to pre-Covid capacities, I don't think getting a reservation for any park will be an issue outside of holidays. Pre-Covid, it was rare for a park to hit capacity outside of the Christmas/NYE period. And with the reservation system, people won't find out last-minute that they can't get in at the peak periods- they'll know in advance. And any other time of year, it shouldn't be an issue to get a park reservation on the same day.
I rarely plan my trips in advance, and I haven't had an issue with park reservations other than one time when a friend told me he was going to HS the night before. I couldn't get a reservation then, but have all the other times I've tried.