If you won't be 21 during this trip, then I wouldn't bother going to Las Vegas except maybe to take a quick drive down the strip. You have to be 21 to even enter the casino complexes, so what you could do would be very limiting. Even in other cities, you won't be able to go out to clubs or bars so you'd probably be best off waiting until everyone is 21.

Also, being under 21 would make it impossible for you to get a rental car.

I had to each 3-4 hours of road a 30minute break, This way we will all be still fresh to sight see the day after.
No offense, but If you think you would be able to drive from Memphis to San Diego with just 30 minutes breaks here and there and make the trip safely, I think you are really fooling yourself. If you think you can take that same approach, get there safely, AND still be "fresh to sight see the day after," I think you are severely misjudging the limits of the human body. You need real sleep to be ready to safely drive, and most people can't get that quality sleep sitting in a moving car.

As has been said on this forum many times, most veteran roadtrippers have taken one shot trips of 1000 or so miles, and no one who has done it recommends it, especially if you need to get on the road at all for a 2nd day. In the best case, its a recipe to have a very tired and cranky group of travelers, in the worst case, its a recipe to cause serious harm to yourself or others on the road.