We have a pseudo-tanning ledge in ours. Our second step is 48" wide and the water depth is about 16", which is wonderful for us. Perfect for a $20 resin Adirondack chair, great for smaller children, perfect for me to stand and throw the football to our kids. I love vinyl over steps, it looks good, feels good, and is just nice. My steps are custom built with concrete block, gravel, and vermiculite. I would have no issues with a dog getting in. LOL...our golden mix does not like water, which is weird to me. She just lays in the yard and watches us swim.
Your yard is so large, Id go with a 20'x40', or maybe even a nice, big "L" shaped pool that gives you a large shallow end, and you can also put a deep end and diving board. We were space limited, so our deep end is a little under six feet.