Sonny, I've never done anything exactly like what you are contemplating; but you have heard correctly: the traffic can be horrendous. But that is usually on Friday and Sunday afternoons and holiday weekends. If you start early enough Sunday morning, October 4th, it shouldn't be too bad.
Another option you might consider: check in the rental car in Oceanside and take a Super Shuttle to the pier. It would be very reasonable for only 40 miles, would save one round trip, and lessen the chances of getting tied up in traffic. Also, the resort in Oceanside may even offer a complimentary or reduced fare shuttle.