I can accept most of your explanation, but if this entire scene is symbolic of creation, one would expect the symbols used to be of the natural world. And for the most part, they are. But the Djed is not. In fact, not only is an artificial construct, but it is either abstract or functional in a way not properly understood. It is not Graeco but much older and has been included for a specific reason.
I am struggling to understand how the Djed can, in your words, "make the sunrise last eternal." I'm not saying that it is an insulator, although a comparison is obvious, but it does touch the "tube" which you say is the sky including the sun. Somehow, I feel that there is more to the Djed than just a symbol of an eternal process. What would be, is a good question, but it isn't convincing enough for me at the moment.
Do you think that a Djed could have had any practical purpose as well as a symbolic one?