On, “If you could design a religion . . . ” -> Have you considered the possibility that beliefs themselves are their own stumbling blocks? Once you believe something, you interpret the world through that lens, which to further beliefs and so on. Exempli gratia: If, like our ancestors, you believe that the sun has been eaten by a dragon, and you decide to try and frighten it away with noise or arrows – with the amazing result of the sun's return – you end up believing that every eclipse you need to shoot some arrows. Or bang on some drums. So strong are these beliefs that these traditions persist in some cultures to this day! If I were to design a religion, it would not contain beliefs. It would simply be a method of developing – and testing! always testing! – working knowledge, and never assuming that any piece of knowledge is absolute. I know this sounds a lot like science, and I guess it is – so my answer to the question is really just for those who can't let go of the need to believe – a placeholder while useful knowledge can still progress.
-Q