I'd go 3/4 or thicker for the tread and put a cleat under the ends and a truss under each step. As you shift you weight upward, you are really loading each step, Another option is to add steel under each step to keep the sides from spreading outward. A half inch dado doesn't have much of a grip. You could also anchor the step by putting a couple of Kreg pocket hole screws through the top of each tread, into the long sides. I'd consider making the steps from inch thick Oak treads you can buy at HD. This thing is going to be heavy.
The more I do, the less I accomplish.