Welcome to the RTA Forum!

A couple things.

First while you've listed 2 "???" days between the Grand Canyon, you'd need at least one of them just to travel between GC and SD. If you only want to travel 5 hours a day, plus you want to travel some of old route 66, you'll certainly need both of those days for the trip.

You should also know that Route 66 actually doesn't exist anymore, as the highway was decommissioned more than 2 decades ago. However, there are some stretches of the old road that you could take in Arizona as you make your way towards San Diego.

I'll also say that if you really want beach time, you'd be better off spending more time around San Diego or LA. There are some beaches as you make your way north, but the central coast is more of a rocky area than an area with great beaches. Not to mention, with the cool waters of the Pacific, being farther south will keep you at least a little bit warmer.