hello @bdmomuae,
I think that writing is the most difficult skill among the four skills. The difficulty of writing does not only lie in creating and organizing the idea, but also in translating the idea into readable writing.
My research when I was in university is also about writing skill, that why I can say this skill is much difficult than others.

But in this article you can explain it easily to be understood. Really, I like you analogy between paragraph body and cheeseburger. I think I will use this one when I teach my students about paragraph.