Both of my children attend Cal Poly SLO. One is a Senior, the other is a Freshman. In a non-Covid year (2017) on-campus housing was required and all students had a spot. While it is always a good idea to submit the housing application as soon as you can, it may not make any difference as to whether your student ends up in the apartments, the older dorms, or the new dorms.
Obviously, the new dorms seem like the first choice, and that may be so for many, many students. If your student ends up there, outstanding.
My son was enrolled before the new dorms were finished so he ended up in the Red Bricks. They are the older, traditional-style dorms with the big shared bathroom down the hall. He was fine there and made lots of friends, some of whom he lives with now.
My daughter got caught up in the Covid housing situation this year. She about cried when she ended up in the Cerra Vista apartments. However, two weeks into the term she was loving the apartments (especially because of Covid issues) and knew several people who were trying to move over from all of the other dorms. Her college (C.A.F.E.S.) has a two year live-on requirement, so she is happily going back into the apartments.
It will all work out no matter where you child ends up IF your child makes the best of it. Cal Poly is a wonderful place.