My preference is to design a spa place above the pool at 18" for a several reasons.

The sight aspect of water cascading into the pool. The sound aspect of again, the water cascading into the pool. It can be very relaxing sitting pool side and just listening to the water. It also drowns out (slightly) neighborhood noises.

With the spa raised 18", it provides additional seating. Also, you now have the opportunity to use a really special material around the spa to make it a focal point. Materials such as glass tile or a veneered natural stone add a nice touch.

We generally place the sun deck at the entry point into the pool. I would go the full width of the pool. If your thinking about placing lounge chairs on it, the sun deck must be large enough to support one-8' minimum. Steps off the sun deck into the pool are added to it.

We keep the depth on a sun deck about 6". More than that makes the entry riser too high after coping is added.

Remember too, the sun deck will encroach on the pool's overall length. If you can go longer than 30' it will help. I'm guessing though the pool is a constant depth.

We do a lot of modern designs and the idea, in my opinion, is to keep it simple.